<- RFC Index (4601..4700)
RFC 4646
Obsoletes RFC 3066
Obsoleted by RFC 5646
Network Working Group A. Phillips, Ed.
Request for Comments: 4646 Yahoo! Inc.
BCP: 47 M. Davis, Ed.
Obsoletes: 3066 Google
Category: Best Current Practice September 2006
Tags for Identifying Languages
Status of This Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document describes the structure, content, construction, and
semantics of language tags for use in cases where it is desirable to
indicate the language used in an information object. It also
describes how to register values for use in language tags and the
creation of user-defined extensions for private interchange. This
document, in combination with RFC 4647, replaces RFC 3066, which
replaced RFC 1766.
Phillips & Davis Best Current Practice [Page 1]
RFC 4646 Tags for Identifying Languages September 2006
Table of Contents
1. Introduction ....................................................3
2. The Language Tag ................................................4
2.1. Syntax .....................................................4
2.2. Language Subtag Sources and Interpretation .................7
2.2.1. Primary Language Subtag .............................8
2.2.2. Extended Language Subtags ..........................10
2.2.3. Script Subtag ......................................11
2.2.4. Region Subtag ......................................11
2.2.5. Variant Subtags ....................................13
2.2.6. Extension Subtags ..................................14
2.2.7. Private Use Subtags ................................16
2.2.8. Preexisting RFC 3066 Registrations .................16
2.2.9. Classes of Conformance .............................17
3. Registry Format and Maintenance ................................18
3.1. Format of the IANA Language Subtag Registry ...............18
3.2. Language Subtag Reviewer ..................................24
3.3. Maintenance of the Registry ...............................24
3.4. Stability of IANA Registry Entries ........................25
3.5. Registration Procedure for Subtags ........................29
3.6. Possibilities for Registration ............................32
3.7. Extensions and Extensions Registry ........................34
3.8. Initialization of the Registries ..........................37
4. Formation and Processing of Language Tags ......................38
4.1. Choice of Language Tag ....................................38
4.2. Meaning of the Language Tag ...............................40
4.3. Length Considerations .....................................41
4.3.1. Working with Limited Buffer Sizes ..................42
4.3.2. Truncation of Language Tags ........................43
4.4. Canonicalization of Language Tags .........................44
4.5. Considerations for Private Use Subtags ....................45
5. IANA Considerations ............................................46
5.1. Language Subtag Registry ..................................46
5.2. Extensions Registry .......................................47
6. Security Considerations ........................................48
7. Character Set Considerations ...................................48
8. Changes from RFC 3066 ..........................................49
9. References .....................................................52
9.1. Normative References ......................................52
9.2. Informative References ....................................53
Appendix A. Acknowledgements ......................................55
Appendix B. Examples of Language Tags (Informative) ...............56
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RFC 4646 Tags for Identifying Languages September 2006
1. Introduction
Human beings on our planet have, past and present, used a number of
languages. There are many reasons why one would want to identify the
language used when presenting or requesting information.
A user's language preferences often need to be identified so that
appropriate processing can be applied. For example, the user's
language preferences in a Web browser can be used to select Web pages
appropriately. Language preferences can also be used to select among
tools (such as dictionaries) to assist in the processing or
understanding of content in different languages.
In addition, knowledge about the particular language used by some
piece of information content might be useful or even required by some
types of processing; for example, spell-checking, computer-
synthesized speech, Braille transcription, or high-quality print
renderings.
One means of indicating the language used is by labeling the
information content with an identifier or "tag". These tags can be
used to specify user preferences when selecting information content,
or for labeling additional attributes of content and associated
resources.
Tags can also be used to indicate additional language attributes of
content. For example, indicating specific information about the
dialect, writing system, or orthography used in a document or
resource may enable the user to obtain information in a form that
they can understand, or it can be important in processing or
rendering the given content into an appropriate form or style.
This document specifies a particular identifier mechanism (the
language tag) and a registration function for values to be used to
form tags. It also defines a mechanism for private use values and
future extension.
This document, in combination with [RFC4647], replaces [RFC3066],
which replaced [RFC1766]. For a list of changes in this document,
see Section 8.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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RFC 4646 Tags for Identifying Languages September 2006
2. The Language Tag
Language tags are used to help identify languages, whether spoken,
written, signed, or otherwise signaled, for the purpose of
communication. This includes constructed and artificial languages,
but excludes languages not intended primarily for human
communication, such as programming languages.
2.1. Syntax
The language tag is composed of one or more parts, known as
"subtags". Each subtag consists of a sequence of alphanumeric
characters. Subtags are distinguished and separated from one another
by a hyphen ("-", ABNF [RFC4234] %x2D). A language tag consists of a
"primary language" subtag and a (possibly empty) series of subsequent
subtags, each of which refines or narrows the range of languages
identified by the overall tag.
Usually, each type of subtag is distinguished by length, position in
the tag, and content: subtags can be recognized solely by these
features. The only exception to this is a fixed list of
grandfathered tags registered under RFC 3066 [RFC3066]. This makes
it possible to construct a parser that can extract and assign some
semantic information to the subtags, even if the specific subtag
values are not recognized. Thus, a parser need not have an up-to-
date copy (or any copy at all) of the subtag registry to perform most
searching and matching operations.
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The syntax of the language tag in ABNF [RFC4234] is:
Language-Tag = langtag
/ privateuse ; private use tag
/ grandfathered ; grandfathered registrations
langtag = (language
["-" script]
["-" region]
*("-" variant)
*("-" extension)
["-" privateuse])
language = (2*3ALPHA [ extlang ]) ; shortest ISO 639 code
/ 4ALPHA ; reserved for future use
/ 5*8ALPHA ; registered language subtag
extlang = *3("-" 3ALPHA) ; reserved for future use
script = 4ALPHA ; ISO 15924 code
region = 2ALPHA ; ISO 3166 code
/ 3DIGIT ; UN M.49 code
variant = 5*8alphanum ; registered variants
/ (DIGIT 3alphanum)
extension = singleton 1*("-" (2*8alphanum))
singleton = %x41-57 / %x59-5A / %x61-77 / %x79-7A / DIGIT
; "a"-"w" / "y"-"z" / "A"-"W" / "Y"-"Z" / "0"-"9"
; Single letters: x/X is reserved for private use
privateuse = ("x"/"X") 1*("-" (1*8alphanum))
grandfathered = 1*3ALPHA 1*2("-" (2*8alphanum))
; grandfathered registration
; Note: i is the only singleton
; that starts a grandfathered tag
alphanum = (ALPHA / DIGIT) ; letters and numbers
Figure 1: Language Tag ABNF
Note: There is a subtlety in the ABNF for 'variant': variants
starting with a digit MAY be four characters long, while those
starting with a letter MUST be at least five characters long.
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All subtags have a maximum length of eight characters and whitespace
is not permitted in a language tag. For examples of language tags,
see Appendix B.
Note that although [RFC4234] refers to octets, the language tags
described in this document are sequences of characters from the
US-ASCII [ISO646] repertoire. Language tags MAY be used in documents
and applications that use other encodings, so long as these encompass
the US-ASCII repertoire. An example of this would be an XML document
that uses the UTF-16LE [RFC2781] encoding of [Unicode].
The tags and their subtags, including private use and extensions, are
to be treated as case insensitive: there exist conventions for the
capitalization of some of the subtags, but these MUST NOT be taken to
carry meaning.
For example:
o [ISO639-1] recommends that language codes be written in lowercase
('mn' Mongolian).
o [ISO3166-1] recommends that country codes be capitalized ('MN'
Mongolia).
o [ISO15924] recommends that script codes use lowercase with the
initial letter capitalized ('Cyrl' Cyrillic).
However, in the tags defined by this document, the uppercase US-ASCII
letters in the range 'A' through 'Z' are considered equivalent and
mapped directly to their US-ASCII lowercase equivalents in the range
'a' through 'z'. Thus, the tag "mn-Cyrl-MN" is not distinct from
"MN-cYRL-mn" or "mN-cYrL-Mn" (or any other combination), and each of
these variations conveys the same meaning: Mongolian written in the
Cyrillic script as used in Mongolia.
Although case distinctions do not carry meaning in language tags,
consistent formatting and presentation of the tags will aid users.
The format of the tags and subtags in the registry is RECOMMENDED.
In this format, all non-initial two-letter subtags are uppercase, all
non-initial four-letter subtags are titlecase, and all other subtags
are lowercase.
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RFC 4646 Tags for Identifying Languages September 2006
2.2. Language Subtag Sources and Interpretation
The namespace of language tags and their subtags is administered by
the Internet Assigned Numbers Authority (IANA) [RFC2860] according to
the rules in Section 5 of this document. The Language Subtag
Registry maintained by IANA is the source for valid subtags: other
standards referenced in this section provide the source material for
that registry.
Terminology in this section:
o Tag or tags refers to a complete language tag, such as
"fr-Latn-CA". Examples of tags in this document are enclosed in
double-quotes ("en-US").
o Subtag refers to a specific section of a tag, delimited by hyphen,
such as the subtag 'Latn' in "fr-Latn-CA". Examples of subtags in
this document are enclosed in single quotes ('Latn').
o Code or codes refers to values defined in external standards (and
that are used as subtags in this document). For example, 'Latn'
is an [ISO15924] script code that was used to define the 'Latn'
script subtag for use in a language tag. Examples of codes in
this document are enclosed in single quotes ('en', 'Latn').
The definitions in this section apply to the various subtags within
the language tags defined by this document, excepting those
"grandfathered" tags defined in Section 2.2.8.
Language tags are designed so that each subtag type has unique length
and content restrictions. These make identification of the subtag's
type possible, even if the content of the subtag itself is
unrecognized. This allows tags to be parsed and processed without
reference to the latest version of the underlying standards or the
IANA registry and makes the associated exception handling when
parsing tags simpler.
Subtags in the IANA registry that do not come from an underlying
standard can only appear in specific positions in a tag.
Specifically, they can only occur as primary language subtags or as
variant subtags.
Note that sequences of private use and extension subtags MUST occur
at the end of the sequence of subtags and MUST NOT be interspersed
with subtags defined elsewhere in this document.
Single-letter and single-digit subtags are reserved for current or
future use. These include the following current uses:
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o The single-letter subtag 'x' is reserved to introduce a sequence
of private use subtags. The interpretation of any private use
subtags is defined solely by private agreement and is not defined
by the rules in this section or in any standard or registry
defined in this document.
o All other single-letter subtags are reserved to introduce
standardized extension subtag sequences as described in
Section 3.7.
The single-letter subtag 'i' is used by some grandfathered tags, such
as "i-enochian", where it always appears in the first position and
cannot be confused with an extension.
2.2.1. Primary Language Subtag
The primary language subtag is the first subtag in a language tag
(with the exception of private use and certain grandfathered tags)
and cannot be omitted. The following rules apply to the primary
language subtag:
1. All two-character language subtags were defined in the IANA
registry according to the assignments found in the standard ISO
639 Part 1, "ISO 639-1:2002, Codes for the representation of
names of languages -- Part 1: Alpha-2 code" [ISO639-1], or using
assignments subsequently made by the ISO 639 Part 1 maintenance
agency or governing standardization bodies.
2. All three-character language subtags were defined in the IANA
registry according to the assignments found in ISO 639 Part 2,
"ISO 639-2:1998 - Codes for the representation of names of
languages -- Part 2: Alpha-3 code - edition 1" [ISO639-2], or
assignments subsequently made by the ISO 639 Part 2 maintenance
agency or governing standardization bodies.
3. The subtags in the range 'qaa' through 'qtz' are reserved for
private use in language tags. These subtags correspond to codes
reserved by ISO 639-2 for private use. These codes MAY be used
for non-registered primary language subtags (instead of using
private use subtags following 'x-'). Please refer to Section 4.5
for more information on private use subtags.
4. All four-character language subtags are reserved for possible
future standardization.
5. All language subtags of 5 to 8 characters in length in the IANA
registry were defined via the registration process in Section 3.5
and MAY be used to form the primary language subtag. At the time
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RFC 4646 Tags for Identifying Languages September 2006
this document was created, there were no examples of this kind of
subtag and future registrations of this type will be discouraged:
primary languages are strongly RECOMMENDED for registration with
ISO 639, and proposals rejected by ISO 639/RA will be closely
scrutinized before they are registered with IANA.
6. The single-character subtag 'x' as the primary subtag indicates
that the language tag consists solely of subtags whose meaning is
defined by private agreement. For example, in the tag "x-fr-CH",
the subtags 'fr' and 'CH' SHOULD NOT be taken to represent the
French language or the country of Switzerland (or any other value
in the IANA registry) unless there is a private agreement in
place to do so. See Section 4.5.
7. The single-character subtag 'i' is used by some grandfathered
tags (see Section 2.2.8) such as "i-klingon" and "i-bnn". (Other
grandfathered tags have a primary language subtag in their first
position.)
8. Other values MUST NOT be assigned to the primary subtag except by
revision or update of this document.
Note: For languages that have both an ISO 639-1 two-character code
and an ISO 639-2 three-character code, only the ISO 639-1 two-
character code is defined in the IANA registry.
Note: For languages that have no ISO 639-1 two-character code and for
which the ISO 639-2/T (Terminology) code and the ISO 639-2/B
(Bibliographic) codes differ, only the Terminology code is defined in
the IANA registry. At the time this document was created, all
languages that had both kinds of three-character code were also
assigned a two-character code; it is not expected that future
assignments of this nature will occur.
Note: To avoid problems with versioning and subtag choice as
experienced during the transition between RFC 1766 and RFC 3066, as
well as the canonical nature of subtags defined by this document, the
ISO 639 Registration Authority Joint Advisory Committee (ISO 639/
RA-JAC) has included the following statement in [iso639.prin]:
"A language code already in ISO 639-2 at the point of freezing ISO
639-1 shall not later be added to ISO 639-1. This is to ensure
consistency in usage over time, since users are directed in Internet
applications to employ the alpha-3 code when an alpha-2 code for that
language is not available."
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In order to avoid instability in the canonical form of tags, if a
two-character code is added to ISO 639-1 for a language for which a
three-character code was already included in ISO 639-2, the two-
character code MUST NOT be registered. See Section 3.4.
For example, if some content were tagged with 'haw' (Hawaiian), which
currently has no two-character code, the tag would not be invalidated
if ISO 639-1 were to assign a two-character code to the Hawaiian
language at a later date.
For example, one of the grandfathered IANA registrations is
"i-enochian". The subtag 'enochian' could be registered in the IANA
registry as a primary language subtag (assuming that ISO 639 does not
register this language first), making tags such as "enochian-AQ" and
"enochian-Latn" valid.
2.2.2. Extended Language Subtags
The following rules apply to the extended language subtags:
1. Three-letter subtags immediately following the primary subtag are
reserved for future standardization, anticipating work that is
currently under way on ISO 639.
2. Extended language subtags MUST follow the primary subtag and
precede any other subtags.
3. There MAY be up to three extended language subtags.
4. Extended language subtags MUST NOT be registered or used to form
language tags. Their syntax is described here so that
implementations can be compatible with any future revision of
this document that does provide for their registration.
Extended language subtag records, once they appear in the registry,
MUST include exactly one 'Prefix' field indicating an appropriate
language subtag or sequence of subtags that MUST always appear as a
prefix to the extended language subtag.
Example: In a future revision or update of this document, the tag
"zh-gan" (registered under RFC 3066) might become a valid non-
grandfathered (that is, redundant) tag in which the subtag 'gan'
might represent the Chinese dialect 'Gan'.
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2.2.3. Script Subtag
Script subtags are used to indicate the script or writing system
variations that distinguish the written forms of a language or its
dialects. The following rules apply to the script subtags:
1. All four-character subtags were defined according to
[ISO15924]--"Codes for the representation of names of scripts":
alpha-4 script codes, or subsequently assigned by the ISO 15924
maintenance agency or governing standardization bodies, denoting
the script or writing system used in conjunction with this
language.
2. Script subtags MUST immediately follow the primary language
subtag and all extended language subtags and MUST occur before
any other type of subtag described below.
3. The script subtags 'Qaaa' through 'Qabx' are reserved for private
use in language tags. These subtags correspond to codes reserved
by ISO 15924 for private use. These codes MAY be used for non-
registered script values. Please refer to Section 4.5 for more
information on private use subtags.
4. Script subtags MUST NOT be registered using the process in
Section 3.5 of this document. Variant subtags MAY be considered
for registration for that purpose.
5. There MUST be at most one script subtag in a language tag, and
the script subtag SHOULD be omitted when it adds no
distinguishing value to the tag or when the primary language
subtag's record includes a Suppress-Script field listing the
applicable script subtag.
Example: "sr-Latn" represents Serbian written using the Latin script.
2.2.4. Region Subtag
Region subtags are used to indicate linguistic variations associated
with or appropriate to a specific country, territory, or region.
Typically, a region subtag is used to indicate regional dialects or
usage, or region-specific spelling conventions. A region subtag can
also be used to indicate that content is expressed in a way that is
appropriate for use throughout a region, for instance, Spanish
content tailored to be useful throughout Latin America.
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The following rules apply to the region subtags:
1. Region subtags MUST follow any language, extended language, or
script subtags and MUST precede all other subtags.
2. All two-character subtags following the primary subtag were
defined in the IANA registry according to the assignments found
in [ISO3166-1] ("Codes for the representation of names of
countries and their subdivisions -- Part 1: Country codes") using
the list of alpha-2 country codes, or using assignments
subsequently made by the ISO 3166 maintenance agency or governing
standardization bodies.
3. All three-character subtags consisting of digit (numeric)
characters following the primary subtag were defined in the IANA
registry according to the assignments found in UN Standard
Country or Area Codes for Statistical Use [UN_M.49] or
assignments subsequently made by the governing standards body.
Note that not all of the UN M.49 codes are defined in the IANA
registry. The following rules define which codes are entered
into the registry as valid subtags:
A. UN numeric codes assigned to 'macro-geographical
(continental)' or sub-regions MUST be registered in the
registry. These codes are not associated with an assigned
ISO 3166 alpha-2 code and represent supra-national areas,
usually covering more than one nation, state, province, or
territory.
B. UN numeric codes for 'economic groupings' or 'other
groupings' MUST NOT be registered in the IANA registry and
MUST NOT be used to form language tags.
C. UN numeric codes for countries or areas with ambiguous ISO
3166 alpha-2 codes, when entered into the registry, MUST be
defined according to the rules in Section 3.4 and MUST be
used to form language tags that represent the country or
region for which they are defined.
D. UN numeric codes for countries or areas for which there is an
associated ISO 3166 alpha-2 code in the registry MUST NOT be
entered into the registry and MUST NOT be used to form
language tags. Note that the ISO 3166-based subtag in the
registry MUST actually be associated with the UN M.49 code in
question.
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E. UN numeric codes and ISO 3166 alpha-2 codes for countries or
areas listed as eligible for registration in [RFC4645] but
not presently registered MAY be entered into the IANA
registry via the process described in Section 3.5. Once
registered, these codes MAY be used to form language tags.
F. All other UN numeric codes for countries or areas that do not
have an associated ISO 3166 alpha-2 code MUST NOT be entered
into the registry and MUST NOT be used to form language tags.
For more information about these codes, see Section 3.4.
4. Note: The alphanumeric codes in Appendix X of the UN document
MUST NOT be entered into the registry and MUST NOT be used to
form language tags. (At the time this document was created,
these values matched the ISO 3166 alpha-2 codes.)
5. There MUST be at most one region subtag in a language tag and the
region subtag MAY be omitted, as when it adds no distinguishing
value to the tag.
6. The region subtags 'AA', 'QM'-'QZ', 'XA'-'XZ', and 'ZZ' are
reserved for private use in language tags. These subtags
correspond to codes reserved by ISO 3166 for private use. These
codes MAY be used for private use region subtags (instead of
using a private use subtag sequence). Please refer to
Section 4.5 for more information on private use subtags.
"de-CH" represents German ('de') as used in Switzerland ('CH').
"sr-Latn-CS" represents Serbian ('sr') written using Latin script
('Latn') as used in Serbia and Montenegro ('CS').
"es-419" represents Spanish ('es') appropriate to the UN-defined
Latin America and Caribbean region ('419').
2.2.5. Variant Subtags
Variant subtags are used to indicate additional, well-recognized
variations that define a language or its dialects that are not
covered by other available subtags. The following rules apply to the
variant subtags:
1. Variant subtags are not associated with any external standard.
Variant subtags and their meanings are defined by the
registration process defined in Section 3.5.
2. Variant subtags MUST follow all of the other defined subtags, but
precede any extension or private use subtag sequences.
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3. More than one variant MAY be used to form the language tag.
4. Variant subtags MUST be registered with IANA according to the
rules in Section 3.5 of this document before being used to form
language tags. In order to distinguish variants from other types
of subtags, registrations MUST meet the following length and
content restrictions:
1. Variant subtags that begin with a letter (a-z, A-Z) MUST be
at least five characters long.
2. Variant subtags that begin with a digit (0-9) MUST be at
least four characters long.
Variant subtag records in the language subtag registry MAY include
one or more 'Prefix' fields, which indicate the language tag or tags
that would make a suitable prefix (with other subtags, as
appropriate) in forming a language tag with the variant. For
example, the subtag 'nedis' has a Prefix of "sl", making it suitable
to form language tags such as "sl-nedis" and "sl-IT-nedis", but not
suitable for use in a tag such as "zh-nedis" or "it-IT-nedis".
"sl-nedis" represents the Natisone or Nadiza dialect of Slovenian.
"de-CH-1996" represents German as used in Switzerland and as written
using the spelling reform beginning in the year 1996 C.E.
Most variants that share a prefix are mutually exclusive. For
example, the German orthographic variations '1996' and '1901' SHOULD
NOT be used in the same tag, as they represent the dates of different
spelling reforms. A variant that can meaningfully be used in
combination with another variant SHOULD include a 'Prefix' field in
its registry record that lists that other variant. For example, if
another German variant 'example' were created that made sense to use
with '1996', then 'example' should include two Prefix fields: "de"
and "de-1996".
2.2.6. Extension Subtags
Extensions provide a mechanism for extending language tags for use in
various applications. See Section 3.7. The following rules apply to
extensions:
1. Extension subtags are separated from the other subtags defined
in this document by a single-character subtag ("singleton").
The singleton MUST be one allocated to a registration authority
via the mechanism described in Section 3.7 and MUST NOT be the
letter 'x', which is reserved for private use subtag sequences.
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2. Note: Private use subtag sequences starting with the singleton
subtag 'x' are described in Section 2.2.7 below.
3. An extension MUST follow at least a primary language subtag.
That is, a language tag cannot begin with an extension.
Extensions extend language tags, they do not override or replace
them. For example, "a-value" is not a well-formed language tag,
while "de-a-value" is.
4. Each singleton subtag MUST appear at most one time in each tag
(other than as a private use subtag). That is, singleton
subtags MUST NOT be repeated. For example, the tag
"en-a-bbb-a-ccc" is invalid because the subtag 'a' appears
twice. Note that the tag "en-a-bbb-x-a-ccc" is valid because
the second appearance of the singleton 'a' is in a private use
sequence.
5. Extension subtags MUST meet all of the requirements for the
content and format of subtags defined in this document.
6. Extension subtags MUST meet whatever requirements are set by the
document that defines their singleton prefix and whatever
requirements are provided by the maintaining authority.
7. Each extension subtag MUST be from two to eight characters long
and consist solely of letters or digits, with each subtag
separated by a single '-'.
8. Each singleton MUST be followed by at least one extension
subtag. For example, the tag "tlh-a-b-foo" is invalid because
the first singleton 'a' is followed immediately by another
singleton 'b'.
9. Extension subtags MUST follow all language, extended language,
script, region, and variant subtags in a tag.
10. All subtags following the singleton and before another singleton
are part of the extension. Example: In the tag "fr-a-Latn", the
subtag 'Latn' does not represent the script subtag 'Latn'
defined in the IANA Language Subtag Registry. Its meaning is
defined by the extension 'a'.
11. In the event that more than one extension appears in a single
tag, the tag SHOULD be canonicalized as described in
Section 4.4.
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For example, if the prefix singleton 'r' and the shown subtags were
defined, then the following tag would be a valid example:
"en-Latn-GB-boont-r-extended-sequence-x-private".
2.2.7. Private Use Subtags
Private use subtags are used to indicate distinctions in language
important in a given context by private agreement. The following
rules apply to private use subtags:
1. Private use subtags are separated from the other subtags defined
in this document by the reserved single-character subtag 'x'.
2. Private use subtags MUST conform to the format and content
constraints defined in the ABNF for all subtags.
3. Private use subtags MUST follow all language, extended language,
script, region, variant, and extension subtags in the tag.
Another way of saying this is that all subtags following the
singleton 'x' MUST be considered private use. Example: The
subtag 'US' in the tag "en-x-US" is a private use subtag.
4. A tag MAY consist entirely of private use subtags.
5. No source is defined for private use subtags. Use of private use
subtags is by private agreement only.
6. Private use subtags are NOT RECOMMENDED where alternatives exist
or for general interchange. See Section 4.5 for more information
on private use subtag choice.
For example: Users who wished to utilize codes from the Ethnologue
publication of SIL International for language identification might
agree to exchange tags such as "az-Arab-x-AZE-derbend". This example
contains two private use subtags. The first is 'AZE' and the second
is 'derbend'.
2.2.8. Preexisting RFC 3066 Registrations
Existing IANA-registered language tags from RFC 1766 and/or RFC 3066
maintain their validity. These tags will be maintained in the
registry in records of either the "grandfathered" or "redundant"
type. Grandfathered tags contain one or more subtags that are not
defined in the Language Subtag Registry (see Section 3). Redundant
tags consist entirely of subtags defined above and whose independent
registration is superseded by this document. For more information,
see Section 3.8.
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It is important to note that all language tags formed under the
guidelines in this document were either legal, well-formed tags or
could have been registered under RFC 3066.
2.2.9. Classes of Conformance
Implementations sometimes need to describe their capabilities with
regard to the rules and practices described in this document. There
are two classes of conforming implementations described by this
document: "well-formed" processors and "validating" processors.
Claims of conformance SHOULD explicitly reference one of these
definitions.
An implementation that claims to check for well-formed language tags
MUST:
o Check that the tag and all of its subtags, including extension and
private use subtags, conform to the ABNF or that the tag is on the
list of grandfathered tags.
o Check that singleton subtags that identify extensions do not
repeat. For example, the tag "en-a-xx-b-yy-a-zz" is not well-
formed.
Well-formed processors are strongly encouraged to implement the
canonicalization rules contained in Section 4.4.
An implementation that claims to be validating MUST:
o Check that the tag is well-formed.
o Specify the particular registry date for which the implementation
performs validation of subtags.
o Check that either the tag is a grandfathered tag, or that all
language, script, region, and variant subtags consist of valid
codes for use in language tags according to the IANA registry as
of the particular date specified by the implementation.
o Specify which, if any, extension RFCs as defined in Section 3.7
are supported, including version, revision, and date.
o For any such extensions supported, check that all subtags used in
that extension are valid.
o For variant and extended language subtags, if the registry
contains one or more 'Prefix' fields for that subtag, check that
the tag matches at least one prefix. The tag matches if all the
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subtags in the 'Prefix' also appear in the tag. For example, the
prefix "es-CO" matches the tag "es-Latn-CO-x-private" because both
the 'es' language subtag and 'CO' region subtag appear in the tag.
3. Registry Format and Maintenance
This section defines the Language Subtag Registry and the maintenance
and update procedures associated with it, as well as a registry for
extensions to language tags (Section 3.7).
The Language Subtag Registry contains a comprehensive list of all of
the subtags valid in language tags. This allows implementers a
straightforward and reliable way to validate language tags. The
Language Subtag Registry will be maintained so that, except for
extension subtags, it is possible to validate all of the subtags that
appear in a language tag under the provisions of this document or its
revisions or successors. In addition, the meaning of the various
subtags will be unambiguous and stable over time. (The meaning of
private use subtags, of course, is not defined by the IANA registry.)
3.1. Format of the IANA Language Subtag Registry
The IANA Language Subtag Registry ("the registry") consists of a text
file that is machine readable in the format described in this
section, plus copies of the registration forms approved in accordance
with the process described in Section 3.5. The existing registration
forms for grandfathered and redundant tags taken from RFC 3066 will
be maintained as part of the obsolete RFC 3066 registry. The
remaining set of initial subtags will not have registration forms
created for them.
The registry is in the text format described below. This format was
based on the record-jar format described in [record-jar].
Each line of text is limited to 72 characters, including all
whitespace. Records are separated by lines containing only the
sequence "%%" (%x25.25).
Each field can be viewed as a single, logical line of ASCII
characters, comprising a field-name and a field-body separated by a
COLON character (%x3A). For convenience, the field-body portion of
this conceptual entity can be split into a multiple-line
representation; this is called "folding". The format of the registry
is described by the following ABNF (per [RFC4234]):
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registry = record *("%%" CRLF record)
record = 1*( field-name *SP ":" *SP field-body CRLF )
field-name = (ALPHA / DIGIT) [*(ALPHA / DIGIT / "-") (ALPHA / DIGIT)]
field-body = *(ASCCHAR/LWSP)
ASCCHAR = %x21-25 / %x27-7E / UNICHAR ; Note: AMPERSAND is %x26
UNICHAR = "&#x" 2*6HEXDIG ";"
Figure 2: Registry Format ABNF
The sequence '..' (%x2E.2E) in a field-body denotes a range of
values. Such a range represents all subtags of the same length that
are in alphabetic or numeric order within that range, including the
values explicitly mentioned. For example 'a..c' denotes the values
'a', 'b', and 'c' and '11..13' denotes the values '11', '12', and
'13'.
Characters from outside the US-ASCII [ISO646] repertoire, as well as
the AMPERSAND character ("&", %x26) when it occurs in a field-body,
are represented by a "Numeric Character Reference" using hexadecimal
notation in the style used by [XML10] (see
<http://www.w3.org/TR/REC-xml/#dt-charref>). This consists of the
sequence "&#x" (%x26.23.78) followed by a hexadecimal representation
of the character's code point in [ISO10646] followed by a closing
semicolon (%x3B). For example, the EURO SIGN, U+20AC, would be
represented by the sequence "€". Note that the hexadecimal
notation MAY have between two and six digits.
All fields whose field-body contains a date value use the "full-date"
format specified in [RFC3339]. For example: "2004-06-28" represents
June 28, 2004, in the Gregorian calendar.
The first record in the file contains the single field whose field-
name is "File-Date" (see Figure 3). The field-body of this record
contains the last modification date of this copy of the registry,
making it possible to compare different versions of the registry.
The registry on the IANA website is the most current. Versions with
an older date than that one are not up-to-date.
File-Date: 2004-06-28
%%
Figure 3: Example of the File-Date Record
Subsequent records represent subtags in the registry. Each of the
fields in each record MUST occur no more than once, unless otherwise
noted below. Each record MUST contain the following fields:
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o 'Type'
* Type's field-value MUST consist of one of the following
strings: "language", "extlang", "script", "region", "variant",
"grandfathered", and "redundant" and denotes the type of tag or
subtag.
o Either 'Subtag' or 'Tag'
* Subtag's field-value contains the subtag being defined. This
field MUST only appear in records of whose 'Type' has one of
these values: "language", "extlang", "script", "region", or
"variant".
* Tag's field-value contains a complete language tag. This field
MUST only appear in records whose 'Type' has one of these
values: "grandfathered" or "redundant". Note that the field-
value will always follow the 'grandfathered' production in the
ABNF in Section 2.1
o Description
* Description's field-value contains a non-normative description
of the subtag or tag.
o Added
* Added's field-value contains the date the record was added to
the registry.
The 'Subtag' or 'Tag' field MUST use lowercase letters to form the
subtag or tag, with two exceptions. Subtags whose 'Type' field is
'script' (in other words, subtags defined by ISO 15924) MUST use
titlecase. Subtags whose 'Type' field is 'region' (in other words,
subtags defined by ISO 3166) MUST use uppercase. These exceptions
mirror the use of case in the underlying standards.
The field 'Description' MAY appear more than one time and contains a
description of the tag or subtag in the record. At least one of the
'Description' fields MUST be written or transcribed into the Latin
script; the same or additional fields MAY also include a description
in a non-Latin script. The 'Description' field is used for
identification purposes and SHOULD NOT be taken to represent the
actual native name of the language or variation or to be in any
particular language. Most descriptions are taken directly from
source standards such as ISO 639 or ISO 3166.
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Note: Descriptions in registry entries that correspond to ISO 639,
ISO 15924, ISO 3166, or UN M.49 codes are intended only to indicate
the meaning of that identifier as defined in the source standard at
the time it was added to the registry. The description does not
replace the content of the source standard itself. The descriptions
are not intended to be the English localized names for the subtags.
Localization or translation of language tag and subtag descriptions
is out of scope of this document.
Each record MAY also contain the following fields:
o Preferred-Value
* For fields of type 'language', 'extlang', 'script', 'region',
and 'variant', 'Preferred-Value' contains the subtag of the
same 'Type' that is preferred for forming the language tag.
* For fields of type 'grandfathered' and 'redundant', a canonical
mapping to a complete language tag.
o Deprecated
* Deprecated's field-value contains the date the record was
deprecated.
o Prefix
* Prefix's field-value contains a language tag with which this
subtag MAY be used to form a new language tag, perhaps with
other subtags as well. This field MUST only appear in records
whose 'Type' field-value is 'variant' or 'extlang'. For
example, the 'Prefix' for the variant 'nedis' is 'sl', meaning
that the tags "sl-nedis" and "sl-IT-nedis" might be appropriate
while the tag "is-nedis" is not.
o Comments
* Comments contains additional information about the subtag, as
deemed appropriate for understanding the registry and
implementing language tags using the subtag or tag.
o Suppress-Script
* Suppress-Script contains a script subtag that SHOULD NOT be
used to form language tags with the associated primary language
subtag. This field MUST only appear in records whose 'Type'
field-value is 'language'. See Section 4.1.
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The field 'Deprecated' MAY be added to any record via the maintenance
process described in Section 3.3 or via the registration process
described in Section 3.5. Usually, the addition of a 'Deprecated'
field is due to the action of one of the standards bodies, such as
ISO 3166, withdrawing a code. In some historical cases, it might not
have been possible to reconstruct the original deprecation date. For
these cases, an approximate date appears in the registry. Although
valid in language tags, subtags and tags with a 'Deprecated' field
are deprecated and validating processors SHOULD NOT generate these
subtags. Note that a record that contains a 'Deprecated' field and
no corresponding 'Preferred-Value' field has no replacement mapping.
The field 'Preferred-Value' contains a mapping between the record in
which it appears and another tag or subtag. The value in this field
is STRONGLY RECOMMENDED as the best choice to represent the value of
this record when selecting a language tag. These values form three
groups:
1. ISO 639 language codes that were later withdrawn in favor of
other codes. These values are mostly a historical curiosity.
2. ISO 3166 region codes that have been withdrawn in favor of a new
code. This sometimes happens when a country changes its name or
administration in such a way that warrants a new region code.
3. Tags grandfathered from RFC 3066. In many cases, these tags have
become obsolete because the values they represent were later
encoded by ISO 639.
Records that contain a 'Preferred-Value' field MUST also have a
'Deprecated' field. This field contains a date of deprecation.
Thus, a language tag processor can use the registry to construct the
valid, non-deprecated set of subtags for a given date. In addition,
for any given tag, a processor can construct the set of valid
language tags that correspond to that tag for all dates up to the
date of the registry. The ability to do these mappings MAY be
beneficial to applications that are matching, selecting, for
filtering content based on its language tags.
Note that 'Preferred-Value' mappings in records of type 'region'
sometimes do not represent exactly the same meaning as the original
value. There are many reasons for a country code to be changed, and
the effect this has on the formation of language tags will depend on
the nature of the change in question.
In particular, the 'Preferred-Value' field does not imply retagging
content that uses the affected subtag.
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The field 'Preferred-Value' MUST NOT be modified once created in the
registry. The field MAY be added to records of type "grandfathered"
and "region" according to the rules in Section 3.3. Otherwise the
field MUST NOT be added to any record already in the registry.
The 'Preferred-Value' field in records of type "grandfathered" and
"redundant" contains whole language tags that are strongly
RECOMMENDED for use in place of the record's value. In many cases,
the mappings were created by deprecation of the tags during the
period before this document was adopted. For example, the tag
"no-nyn" was deprecated in favor of the ISO 639-1-defined language
code 'nn'.
Records of type 'variant' MAY have more than one field of type
'Prefix'. Additional fields of this type MAY be added to a 'variant'
record via the registration process.
Records of type 'extlang' MUST have _exactly_ one 'Prefix' field.
The field-value of the 'Prefix' field consists of a language tag
whose subtags are appropriate to use with this subtag. For example,
the variant subtag '1996' has a 'Prefix' field of "de". This means
that tags starting with the sequence "de-" are appropriate with this
subtag, so "de-Latg-1996" and "de-CH-1996" are both acceptable, while
the tag "fr-1996" is an inappropriate choice.
The field of type 'Prefix' MUST NOT be removed from any record. The
field-value for this type of field MUST NOT be modified.
The field 'Comments' MAY appear more than once per record. This
field MAY be inserted or changed via the registration process and no
guarantee of stability is provided. The content of this field is not
restricted, except by the need to register the information, the
suitability of the request, and by reasonable practical size
limitations.
The field 'Suppress-Script' MUST only appear in records whose 'Type'
field-value is 'language'. This field MUST NOT appear more than one
time in a record. This field indicates a script used to write the
overwhelming majority of documents for the given language and that
therefore adds no distinguishing information to a language tag. It
helps ensure greater compatibility between the language tags
generated according to the rules in this document and language tags
and tag processors or consumers based on RFC 3066. For example,
virtually all Icelandic documents are written in the Latin script,
making the subtag 'Latn' redundant in the tag "is-Latn".
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3.2. Language Subtag Reviewer
The Language Subtag Reviewer is appointed by the IESG for an
indefinite term, subject to removal or replacement at the IESG's
discretion. The Language Subtag Reviewer moderates the ietf-
languages mailing list, responds to requests for registration, and
performs the other registry maintenance duties described in
Section 3.3. Only the Language Subtag Reviewer is permitted to
request IANA to change, update, or add records to the Language Subtag
Registry.
The performance or decisions of the Language Subtag Reviewer MAY be
appealed to the IESG under the same rules as other IETF decisions
(see [RFC2026]). The IESG can reverse or overturn the decision of
the Language Subtag Reviewer, provide guidance, or take other
appropriate actions.
3.3. Maintenance of the Registry
Maintenance of the registry requires that as codes are assigned or
withdrawn by ISO 639, ISO 15924, ISO 3166, and UN M.49, the Language
Subtag Reviewer MUST evaluate each change, determine whether it
conflicts with existing registry entries, and submit the information
to IANA for inclusion in the registry. If a change takes place and
the Language Subtag Reviewer does not do this in a timely manner,
then any interested party MAY use the procedure in Section 3.5 to
register the appropriate update.
Note: The redundant and grandfathered entries together are the
complete list of tags registered under [RFC3066]. The redundant tags
are those that can now be formed using the subtags defined in the
registry together with the rules of Section 2.2. The grandfathered
entries include those that can never be legal under those same
provisions.
The set of redundant and grandfathered tags is permanent and stable:
new entries in this section MUST NOT be added and existing entries
MUST NOT be removed. Records of type 'grandfathered' MAY have their
type converted to 'redundant'; see item 12 in Section 3.6 for more
information. The decision-making process about which tags were
initially grandfathered and which were made redundant is described in
[RFC4645].
RFC 3066 tags that were deprecated prior to the adoption of this
document are part of the list of grandfathered tags, and their
component subtags were not included as registered variants (although
they remain eligible for registration). For example, the tag
"art-lojban" was deprecated in favor of the language subtag 'jbo'.
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The Language Subtag Reviewer MUST ensure that new subtags meet the
requirements in Section 4.1 or submit an appropriate alternate subtag
as described in that section. When either a change or addition to
the registry is needed, the Language Subtag Reviewer MUST prepare the
complete record, including all fields, and forward it to IANA for
insertion into the registry. Each record being modified or inserted
MUST be forwarded in a separate message.
If a record represents a new subtag that does not currently exist in
the registry, then the message's subject line MUST include the word
"INSERT". If the record represents a change to an existing subtag,
then the subject line of the message MUST include the word "MODIFY".
The message MUST contain both the record for the subtag being
inserted or modified and the new File-Date record. Here is an
example of what the body of the message might contain:
LANGUAGE SUBTAG MODIFICATION
File-Date: 2005-01-02
%%
Type: variant
Subtag: nedis
Description: Natisone dialect
Description: Nadiza dialect
Added: 2003-10-09
Prefix: sl
Comments: This is a comment shown
as an example.
%%
Figure 4: Example of a Language Subtag Modification Form
Whenever an entry is created or modified in the registry, the
'File-Date' record at the start of the registry is updated to reflect
the most recent modification date in the [RFC3339] "full-date"
format.
Before forwarding a new registration to IANA, the Language Subtag
Reviewer MUST ensure that values in the 'Subtag' field match case
according to the description in Section 3.1.
3.4. Stability of IANA Registry Entries
The stability of entries and their meaning in the registry is
critical to the long-term stability of language tags. The rules in
this section guarantee that a specific language tag's meaning is
stable over time and will not change.
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These rules specifically deal with how changes to codes (including
withdrawal and deprecation of codes) maintained by ISO 639, ISO
15924, ISO 3166, and UN M.49 are reflected in the IANA Language
Subtag Registry. Assignments to the IANA Language Subtag Registry
MUST follow the following stability rules:
1. Values in the fields 'Type', 'Subtag', 'Tag', 'Added',
'Deprecated' and 'Preferred-Value' MUST NOT be changed and are
guaranteed to be stable over time.
2. Values in the 'Description' field MUST NOT be changed in a way
that would invalidate previously-existing tags. They MAY be
broadened somewhat in scope, changed to add information, or
adapted to the most common modern usage. For example, countries
occasionally change their official names; a historical example
of this would be "Upper Volta" changing to "Burkina Faso".
3. Values in the field 'Prefix' MAY be added to records of type
'variant' via the registration process.
4. Values in the field 'Prefix' MAY be modified, so long as the
modifications broaden the set of prefixes. That is, a prefix
MAY be replaced by one of its own prefixes. For example, the
prefix "en-US" could be replaced by "en", but not by the
prefixes "en-Latn", "fr", or "en-US-boont". If one of those
prefixes were needed, a new Prefix SHOULD be registered.
5. Values in the field 'Prefix' MUST NOT be removed.
6. The field 'Comments' MAY be added, changed, modified, or removed
via the registration process or any of the processes or
considerations described in this section.
7. The field 'Suppress-Script' MAY be added or removed via the
registration process.
8. Codes assigned by ISO 639, ISO 15924, and ISO 3166 that do not
conflict with existing subtags of the associated type and whose
meaning is not the same as an existing subtag of the same type
are entered into the IANA registry as new records.
9. Codes assigned by ISO 639, ISO 15924, or ISO 3166 that are
withdrawn by their respective maintenance or registration
authority remain valid in language tags. A 'Deprecated' field
containing the date of withdrawal is added to the record. If a
new record of the same type is added that represents a
Phillips & Davis Best Current Practice [Page 26]
RFC 4646 Tags for Identifying Languages September 2006
replacement value, then a 'Preferred-Value' field MAY also be
added. The registration process MAY be used to add comments
about the withdrawal of the code by the respective standard.
Example
The region code 'TL' was assigned to the country 'Timor-
Leste', replacing the code 'TP' (which was assigned to 'East
Timor' when it was under administration by Portugal). The
subtag 'TP' remains valid in language tags, but its record
contains the a 'Preferred-Value' of 'TL' and its field
'Deprecated' contains the date the new code was assigned
('2004-07-06').
10. Codes assigned by ISO 639, ISO 15924, or ISO 3166 that conflict
with existing subtags of the associated type, including subtags
that are deprecated, MUST NOT be entered into the registry. The
following additional considerations apply to subtag values that
are reassigned:
A. For ISO 639 codes, if the newly assigned code's meaning is
not represented by a subtag in the IANA registry, the
Language Subtag Reviewer, as described in Section 3.5, SHALL
prepare a proposal for entering in the IANA registry as soon
as practical a registered language subtag as an alternate
value for the new code. The form of the registered language
subtag will be at the discretion of the Language Subtag
Reviewer and MUST conform to other restrictions on language
subtags in this document.
B. For all subtags whose meaning is derived from an external
standard (i.e., ISO 639, ISO 15924, ISO 3166, or UN M.49),
if a new meaning is assigned to an existing code and the new
meaning broadens the meaning of that code, then the meaning
for the associated subtag MAY be changed to match. The
meaning of a subtag MUST NOT be narrowed, however, as this
can result in an unknown proportion of the existing uses of
a subtag becoming invalid. Note: ISO 639 maintenance
agency/registration authority (MA/RA) has adopted a similar
stability policy.
C. For ISO 15924 codes, if the newly assigned code's meaning is
not represented by a subtag in the IANA registry, the
Language Subtag Reviewer, as described in Section 3.5, SHALL
prepare a proposal for entering in the IANA registry as soon
as practical a registered variant subtag as an alternate
value for the new code. The form of the registered variant
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RFC 4646 Tags for Identifying Languages September 2006
subtag will be at the discretion of the Language Subtag
Reviewer and MUST conform to other restrictions on variant
subtags in this document.
D. For ISO 3166 codes, if the newly assigned code's meaning is
associated with the same UN M.49 code as another 'region'
subtag, then the existing region subtag remains as the
preferred value for that region and no new entry is created.
A comment MAY be added to the existing region subtag
indicating the relationship to the new ISO 3166 code.
E. For ISO 3166 codes, if the newly assigned code's meaning is
associated with a UN M.49 code that is not represented by an
existing region subtag, then the Language Subtag Reviewer,
as described in Section 3.5, SHALL prepare a proposal for
entering the appropriate UN M.49 country code as an entry in
the IANA registry.
F. For ISO 3166 codes, if there is no associated UN numeric
code, then the Language Subtag Reviewer SHALL petition the
UN to create one. If there is no response from the UN
within ninety days of the request being sent, the Language
Subtag Reviewer SHALL prepare a proposal for entering in the
IANA registry as soon as practical a registered variant
subtag as an alternate value for the new code. The form of
the registered variant subtag will be at the discretion of
the Language Subtag Reviewer and MUST conform to other
restrictions on variant subtags in this document. This
situation is very unlikely to ever occur.
11. UN M.49 has codes for both countries and areas (such as '276'
for Germany) and geographical regions and sub-regions (such as
'150' for Europe). UN M.49 country or area codes for which
there is no corresponding ISO 3166 code SHOULD NOT be
registered, except as a surrogate for an ISO 3166 code that is
blocked from registration by an existing subtag. If such a code
becomes necessary, then the registration authority for ISO 3166
SHOULD first be petitioned to assign a code to the region. If
the petition for a code assignment by ISO 3166 is refused or not
acted on in a timely manner, the registration process described
in Section 3.5 MAY then be used to register the corresponding UN
M.49 code. At the time this document was written, there were
only four such codes: 830 (Channel Islands), 831 (Guernsey), 832
(Jersey), and 833 (Isle of Man). This way, UN M.49 codes remain
available as the value of last resort in cases where ISO 3166
reassigns a deprecated value in the registry.
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12. Stability provisions apply to grandfathered tags with this
exception: should all of the subtags in a grandfathered tag
become valid subtags in the IANA registry, then the field 'Type'
in that record is changed from 'grandfathered' to 'redundant'.
Note that this will not affect language tags that match the
grandfathered tag, since these tags will now match valid
generative subtag sequences. For example, if the subtag 'gan'
in the language tag "zh-gan" were to be registered as an
extended language subtag, then the grandfathered tag "zh-gan"
would be deprecated (but existing content or implementations
that use "zh-gan" would remain valid).
3.5. Registration Procedure for Subtags
The procedure given here MUST be used by anyone who wants to use a
subtag not currently in the IANA Language Subtag Registry.
Only subtags of type 'language' and 'variant' will be considered for
independent registration of new subtags. Handling of subtags needed
for stability and subtags necessary to keep the registry synchronized
with ISO 639, ISO 15924, ISO 3166, and UN M.49 within the limits
defined by this document are described in Section 3.3. Stability
provisions are described in Section 3.4.
This procedure MAY also be used to register or alter the information
for the 'Description', 'Comments', 'Deprecated', or 'Prefix' fields
in a subtag's record as described in Section 3.4. Changes to all
other fields in the IANA registry are NOT permitted.
Registering a new subtag or requesting modifications to an existing
tag or subtag starts with the requester filling out the registration
form reproduced below. Note that each response is not limited in
size so that the request can adequately describe the registration.
The fields in the "Record Requested" section SHOULD follow the
requirements in Section 3.1.
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LANGUAGE SUBTAG REGISTRATION FORM
1. Name of requester:
2. E-mail address of requester:
3. Record Requested:
Type:
Subtag:
Description:
Prefix:
Preferred-Value:
Deprecated:
Suppress-Script:
Comments:
4. Intended meaning of the subtag:
5. Reference to published description
of the language (book or article):
6. Any other relevant information:
Figure 5: The Language Subtag Registration Form
The subtag registration form MUST be sent to
<ietf-languages@iana.org> for a two-week review period before it can
be submitted to IANA. (This is an open list and can be joined by
sending a request to <ietf-languages-request@iana.org>.)
Variant subtags are usually registered for use with a particular
range of language tags. For example, the subtag 'rozaj' is intended
for use with language tags that start with the primary language
subtag "sl", since Resian is a dialect of Slovenian. Thus, the
subtag 'rozaj' would be appropriate in tags such as "sl-Latn-rozaj"
or "sl-IT-rozaj". This information is stored in the 'Prefix' field
in the registry. Variant registration requests SHOULD include at
least one 'Prefix' field in the registration form.
Extended language subtags are reserved for future standardization.
These subtags will be REQUIRED to include exactly one 'Prefix' field
once they are allowed for registration.
The 'Prefix' field for a given registered subtag exists in the IANA
registry as a guide to usage. Additional prefixes MAY be added by
filing an additional registration form. In that form, the "Any other
relevant information:" field MUST indicate that it is the addition of
a prefix.
Requests to add a prefix to a variant subtag that imply a different
semantic meaning will probably be rejected. For example, a request
to add the prefix "de" to the subtag 'nedis' so that the tag
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"de-nedis" represented some German dialect would be rejected. The
'nedis' subtag represents a particular Slovenian dialect and the
additional registration would change the semantic meaning assigned to
the subtag. A separate subtag SHOULD be proposed instead.
The 'Description' field MUST contain a description of the tag being
registered written or transcribed into the Latin script; it MAY also
include a description in a non-Latin script. Non-ASCII characters
MUST be escaped using the syntax described in Section 3.1. The
'Description' field is used for identification purposes and doesn't
necessarily represent the actual native name of the language or
variation or to be in any particular language.
While the 'Description' field itself is not guaranteed to be stable
and errata corrections MAY be undertaken from time to time, attempts
to provide translations or transcriptions of entries in the registry
itself will probably be frowned upon by the community or rejected
outright, as changes of this nature have an impact on the provisions
in Section 3.4.
When the two-week period has passed, the Language Subtag Reviewer
either forwards the record to be inserted or modified to
iana@iana.org according to the procedure described in Section 3.3, or
rejects the request because of significant objections raised on the
list or due to problems with constraints in this document (which MUST
be explicitly cited). The Language Subtag Reviewer MAY also extend
the review period in two-week increments to permit further
discussion. The Language Subtag Reviewer MUST indicate on the list
whether the registration has been accepted, rejected, or extended
following each two-week period.
Note that the Language Subtag Reviewer MAY raise objections on the
list if he or she so desires. The important thing is that the
objection MUST be made publicly.
The applicant is free to modify a rejected application with
additional information and submit it again; this restarts the two-
week comment period.
Decisions made by the Language Subtag Reviewer MAY be appealed to the
IESG [RFC2028] under the same rules as other IETF decisions
[RFC2026].
All approved registration forms are available online in the directory
http://www.iana.org/numbers.html under "languages".
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Updates or changes to existing records follow the same procedure as
new registrations. The Language Subtag Reviewer decides whether
there is consensus to update the registration following the two-week
review period; normally, objections by the original registrant will
carry extra weight in forming such a consensus.
Registrations are permanent and stable. Once registered, subtags
will not be removed from the registry and will remain a valid way in
which to specify a specific language or variant.
Note: The purpose of the "Description" in the registration form is to
aid people trying to verify whether a language is registered or what
language or language variation a particular subtag refers to. In
most cases, reference to an authoritative grammar or dictionary of
that language will be useful; in cases where no such work exists,
other well-known works describing that language or in that language
MAY be appropriate. The Language Subtag Reviewer decides what
constitutes "good enough" reference material. This requirement is
not intended to exclude particular languages or dialects due to the
size of the speaker population or lack of a standardized orthography.
Minority languages will be considered equally on their own merits.
3.6. Possibilities for Registration
Possibilities for registration of subtags or information about
subtags include:
o Primary language subtags for languages not listed in ISO 639 that
are not variants of any listed or registered language MAY be
registered. At the time this document was created, there were no
examples of this form of subtag. Before attempting to register a
language subtag, there MUST be an attempt to register the language
with ISO 639. Subtags MUST NOT be registered for codes that exist
in ISO 639-1 or ISO 639-2, that are under consideration by the ISO
639 maintenance or registration authorities, or that have never
been attempted for registration with those authorities. If ISO
639 has previously rejected a language for registration, it is
reasonable to assume that there must be additional, very
compelling evidence of need before it will be registered in the
IANA registry (to the extent that it is very unlikely that any
subtags will be registered of this type).
o Dialect or other divisions or variations within a language, its
orthography, writing system, regional or historical usage,
transliteration or other transformation, or distinguishing
variation MAY be registered as variant subtags. An example is the
'rozaj' subtag (the Resian dialect of Slovenian).
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o The addition or maintenance of fields (generally of an
informational nature) in Tag or Subtag records as described in
Section 3.1 and subject to the stability provisions in
Section 3.4. This includes descriptions, comments, deprecation
and preferred values for obsolete or withdrawn codes, or the
addition of script or extlang information to primary language
subtags.
o The addition of records and related field value changes necessary
to reflect assignments made by ISO 639, ISO 15924, ISO 3166, and
UN M.49 as described in Section 3.4.
Subtags proposed for registration that would cause all or part of a
grandfathered tag to become redundant but whose meaning conflicts
with or alters the meaning of the grandfathered tag MUST be rejected.
This document leaves the decision on what subtags or changes to
subtags are appropriate (or not) to the registration process
described in Section 3.5.
Note: four-character primary language subtags are reserved to allow
for the possibility of alpha4 codes in some future addition to the
ISO 639 family of standards.
ISO 639 defines a maintenance agency for additions to and changes in
the list of languages in ISO 639. This agency is:
International Information Centre for Terminology (Infoterm)
Aichholzgasse 6/12, AT-1120
Wien, Austria
Phone: +43 1 26 75 35 Ext. 312 Fax: +43 1 216 32 72
ISO 639-2 defines a maintenance agency for additions to and changes
in the list of languages in ISO 639-2. This agency is:
Library of Congress
Network Development and MARC Standards Office
Washington, D.C. 20540 USA
Phone: +1 202 707 6237 Fax: +1 202 707 0115
URL: http://www.loc.gov/standards/iso639-2
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The maintenance agency for ISO 3166 (country codes) is:
ISO 3166 Maintenance Agency
c/o International Organization for Standardization
Case postale 56
CH-1211 Geneva 20 Switzerland
Phone: +41 22 749 72 33 Fax: +41 22 749 73 49
URL: http://www.iso.org/iso/en/prods-services/iso3166ma/index.html
The registration authority for ISO 15924 (script codes) is:
Unicode Consortium Box 391476
Mountain View, CA 94039-1476, USA
URL: http://www.unicode.org/iso15924
The Statistics Division of the United Nations Secretariat maintains
the Standard Country or Area Codes for Statistical Use and can be
reached at:
Statistical Services Branch
Statistics Division
United Nations, Room DC2-1620
New York, NY 10017, USA
Fax: +1-212-963-0623
E-mail: statistics@un.org
URL: http://unstats.un.org/unsd/methods/m49/m49alpha.htm
3.7. Extensions and Extensions Registry
Extension subtags are those introduced by single-character subtags
("singletons") other than 'x'. They are reserved for the generation
of identifiers that contain a language component and are compatible
with applications that understand language tags.
The structure and form of extensions are defined by this document so
that implementations can be created that are forward compatible with
applications that might be created using singletons in the future.
In addition, defining a mechanism for maintaining singletons will
lend stability to this document by reducing the likely need for
future revisions or updates.
Single-character subtags are assigned by IANA using the "IETF
Consensus" policy defined by [RFC2434]. This policy requires the
development of an RFC, which SHALL define the name, purpose,
processes, and procedures for maintaining the subtags. The
maintaining or registering authority, including name, contact email,
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discussion list email, and URL location of the registry, MUST be
indicated clearly in the RFC. The RFC MUST specify or include each
of the following:
o The specification MUST reference the specific version or revision
of this document that governs its creation and MUST reference this
section of this document.
o The specification and all subtags defined by the specification
MUST follow the ABNF and other rules for the formation of tags and
subtags as defined in this document. In particular, it MUST
specify that case is not significant and that subtags MUST NOT
exceed eight characters in length.
o The specification MUST specify a canonical representation.
o The specification of valid subtags MUST be available over the
Internet and at no cost.
o The specification MUST be in the public domain or available via a
royalty-free license acceptable to the IETF and specified in the
RFC.
o The specification MUST be versioned, and each version of the
specification MUST be numbered, dated, and stable.
o The specification MUST be stable. That is, extension subtags,
once defined by a specification, MUST NOT be retracted or change
in meaning in any substantial way.
o The specification MUST include in a separate section the
registration form reproduced in this section (below) to be used in
registering the extension upon publication as an RFC.
o IANA MUST be informed of changes to the contact information and
URL for the specification.
IANA will maintain a registry of allocated single-character
(singleton) subtags. This registry MUST use the record-jar format
described by the ABNF in Section 3.1. Upon publication of an
extension as an RFC, the maintaining authority defined in the RFC
MUST forward this registration form to iesg@ietf.org, who MUST
forward the request to iana@iana.org. The maintaining authority of
the extension MUST maintain the accuracy of the record by sending an
updated full copy of the record to iana@iana.org with the subject
line "LANGUAGE TAG EXTENSION UPDATE" whenever content changes. Only
the 'Comments', 'Contact_Email', 'Mailing_List', and 'URL' fields MAY
be modified in these updates.
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Failure to maintain this record, maintain the corresponding registry,
or meet other conditions imposed by this section of this document MAY
be appealed to the IESG [RFC2028] under the same rules as other IETF
decisions (see [RFC2026]) and MAY result in the authority to maintain
the extension being withdrawn or reassigned by the IESG.
%%
Identifier:
Description:
Comments:
Added:
RFC:
Authority:
Contact_Email:
Mailing_List:
URL:
%%
Figure 6: Format of Records in the Language Tag Extensions Registry
'Identifier' contains the single-character subtag (singleton)
assigned to the extension. The Internet-Draft submitted to define
the extension SHOULD specify which letter or digit to use, although
the IESG MAY change the assignment when approving the RFC.
'Description' contains the name and description of the extension.
'Comments' is an OPTIONAL field and MAY contain a broader description
of the extension.
'Added' contains the date the RFC was published in the "full-date"
format specified in [RFC3339]. For example: 2004-06-28 represents
June 28, 2004, in the Gregorian calendar.
'RFC' contains the RFC number assigned to the extension.
'Authority' contains the name of the maintaining authority for the
extension.
'Contact_Email' contains the email address used to contact the
maintaining authority.
'Mailing_List' contains the URL or subscription email address of the
mailing list used by the maintaining authority.
'URL' contains the URL of the registry for this extension.
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The determination of whether an Internet-Draft meets the above
conditions and the decision to grant or withhold such authority rests
solely with the IESG and is subject to the normal review and appeals
process associated with the RFC process.
Extension authors are strongly cautioned that many (including most
well-formed) processors will be unaware of any special relationships
or meaning inherent in the order of extension subtags. Extension
authors SHOULD avoid subtag relationships or canonicalization
mechanisms that interfere with matching or with length restrictions
that sometimes exist in common protocols where the extension is used.
In particular, applications MAY truncate the subtags in doing
matching or in fitting into limited lengths, so it is RECOMMENDED
that the most significant information be in the most significant
(left-most) subtags and that the specification gracefully handle
truncated subtags.
When a language tag is to be used in a specific, known, protocol, it
is RECOMMENDED that the language tag not contain extensions not
supported by that protocol. In addition, note that some protocols
MAY impose upper limits on the length of the strings used to store or
transport the language tag.
3.8. Initialization of the Registries
Upon adoption of this document, an initial version of the Language
Subtag Registry containing the various subtags initially valid in a
language tag is necessary. This collection of subtags, along with a
description of the process used to create it, is described by
[RFC4645]. IANA SHALL publish the initial version of the registry
described by this document from the content of [RFC4645]. Once
published by IANA, the maintenance procedures, rules, and
registration processes described in this document will be available
for new registrations or updates.
Registrations that are in process under the rules defined in
[RFC3066] when this document is adopted MAY be completed under the
former rules, at the discretion of the Language Tag Reviewer (as
described in [RFC3066]). Until the IESG officially appoints a
Language Subtag Reviewer, the existing Language Tag Reviewer SHALL
serve as the Language Subtag Reviewer.
Any new registrations submitted using the RFC 3066 forms or format
after the adoption of this document and publication of the registry
by IANA MUST be rejected.
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An initial version of the Language Tag Extensions Registry described
in Section 3.7 is also needed. The Language Tag Extensions Registry
SHALL be initialized with a single record containing a single field
of type "File-Date" as a placeholder for future assignments.
4. Formation and Processing of Language Tags
This section addresses how to use the information in the registry
with the tag syntax to choose, form, and process language tags.
4.1. Choice of Language Tag
One is sometimes faced with the choice between several possible tags
for the same body of text.
Interoperability is best served when all users use the same language
tag in order to represent the same language. If an application has
requirements that make the rules here inapplicable, then that
application risks damaging interoperability. It is strongly
RECOMMENDED that users not define their own rules for language tag
choice.
Subtags SHOULD only be used where they add useful distinguishing
information; extraneous subtags interfere with the meaning,
understanding, and processing of language tags. In particular, users
and implementations SHOULD follow the 'Prefix' and 'Suppress-Script'
fields in the registry (defined in Section 3.1): these fields provide
guidance on when specific additional subtags SHOULD (and SHOULD NOT)
be used in a language tag.
Of particular note, many applications can benefit from the use of
script subtags in language tags, as long as the use is consistent for
a given context. Script subtags were not formally defined in RFC
3066 and their use can affect matching and subtag identification by
implementations of RFC 3066, as these subtags appear between the
primary language and region subtags. For example, if a user requests
content in an implementation of Section 2.5 of [RFC3066] using the
language range "en-US", content labeled "en-Latn-US" will not match
the request. Therefore, it is important to know when script subtags
will customarily be used and when they ought not be used. In the
registry, the Suppress-Script field helps ensure greater
compatibility between the language tags generated according to the
rules in this document and language tags and tag processors or
consumers based on RFC 3066 by defining when users SHOULD NOT include
a script subtag with a particular primary language subtag.
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Extended language subtags (type 'extlang' in the registry; see
Section 3.1) also appear between the primary language and region
subtags and are reserved for future standardization. Applications
might benefit from their judicious use in forming language tags in
the future. Similar recommendations are expected to apply to their
use as apply to script subtags.
Standards, protocols, and applications that reference this document
normatively but apply different rules to the ones given in this
section MUST specify how the procedure varies from the one given
here.
The choice of subtags used to form a language tag SHOULD be guided by
the following rules:
1. Use as precise a tag as possible, but no more specific than is
justified. Avoid using subtags that are not important for
distinguishing content in an application.
* For example, 'de' might suffice for tagging an email written
in German, while "de-CH-1996" is probably unnecessarily
precise for such a task.
2. The script subtag SHOULD NOT be used to form language tags unless
the script adds some distinguishing information to the tag. The
field 'Suppress-Script' in the primary language record in the
registry indicates which script subtags do not add distinguishing
information for most applications.
* For example, the subtag 'Latn' should not be used with the
primary language 'en' because nearly all English documents are
written in the Latin script and it adds no distinguishing
information. However, if a document were written in English
mixing Latin script with another script such as Braille
('Brai'), then it might be appropriate to choose to indicate
both scripts to aid in content selection, such as the
application of a style sheet.
3. If a tag or subtag has a 'Preferred-Value' field in its registry
entry, then the value of that field SHOULD be used to form the
language tag in preference to the tag or subtag in which the
preferred value appears.
* For example, use 'he' for Hebrew in preference to 'iw'.
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4. The 'und' (Undetermined) primary language subtag SHOULD NOT be
used to label content, even if the language is unknown. Omitting
the language tag altogether is preferred to using a tag with a
primary language subtag of 'und'. The 'und' subtag MAY be useful
for protocols that require a language tag to be provided. The
'und' subtag MAY also be useful when matching language tags in
certain situations.
5. The 'mul' (Multiple) primary language subtag SHOULD NOT be used
whenever the protocol allows the separate tags for multiple
languages, as is the case for the Content-Language header in
HTTP. The 'mul' subtag conveys little useful information:
content in multiple languages SHOULD individually tag the
languages where they appear or otherwise indicate the actual
language in preference to the 'mul' subtag.
6. The same variant subtag SHOULD NOT be used more than once within
a language tag.
* For example, do not use "de-DE-1901-1901".
To ensure consistent backward compatibility, this document contains
several provisions to account for potential instability in the
standards used to define the subtags that make up language tags.
These provisions mean that no language tag created under the rules in
this document will become obsolete.
4.2. Meaning of the Language Tag
The relationship between the tag and the information it relates to is
defined by the context in which the tag appears. Accordingly, this
section gives only possible examples of its usage.
o For a single information object, the associated language tags
might be interpreted as the set of languages that is necessary for
a complete comprehension of the complete object. Example: Plain
text documents.
o For an aggregation of information objects, the associated language
tags could be taken as the set of languages used inside components
of that aggregation. Examples: Document stores and libraries.
o For information objects whose purpose is to provide alternatives,
the associated language tags could be regarded as a hint that the
content is provided in several languages and that one has to
inspect each of the alternatives in order to find its language or
languages. In this case, the presence of multiple tags might not
mean that one needs to be multi-lingual to get complete
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understanding of the document. Example: MIME multipart/
alternative.
o In markup languages, such as HTML and XML, language information
can be added to each part of the document identified by the markup
structure (including the whole document itself). For example, one
could write <span lang="fr">C'est la vie.</span> inside a
Norwegian document; the Norwegian-speaking user could then access
a French-Norwegian dictionary to find out what the marked section
meant. If the user were listening to that document through a
speech synthesis interface, this formation could be used to signal
the synthesizer to appropriately apply French text-to-speech
pronunciation rules to that span of text, instead of applying the
inappropriate Norwegian rules.
Language tags are related when they contain a similar sequence of
subtags. For example, if a language tag B contains language tag A as
a prefix, then B is typically "narrower" or "more specific" than A.
Thus, "zh-Hant-TW" is more specific than "zh-Hant".
This relationship is not guaranteed in all cases: specifically,
languages that begin with the same sequence of subtags are NOT
guaranteed to be mutually intelligible, although they might be. For
example, the tag "az" shares a prefix with both "az-Latn"
(Azerbaijani written using the Latin script) and "az-Cyrl"
(Azerbaijani written using the Cyrillic script). A person fluent in
one script might not be able to read the other, even though the text
might be identical. Content tagged as "az" most probably is written
in just one script and thus might not be intelligible to a reader
familiar with the other script.
4.3. Length Considerations
[RFC3066] did not provide an upper limit on the size of language
tags. While RFC 3066 did define the semantics of particular subtags
in such a way that most language tags consisted of language and
region subtags with a combined total length of up to six characters,
larger registered tags were not only possible but were actually
registered.
Neither the language tag syntax nor other requirements in this
document impose a fixed upper limit on the number of subtags in a
language tag (and thus an upper bound on the size of a tag). The
language tag syntax suggests that, depending on the specific
language, more subtags (and thus a longer tag) are sometimes
necessary to completely identify the language for certain
applications; thus, it is possible to envision long or complex subtag
sequences.
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4.3.1. Working with Limited Buffer Sizes
Some applications and protocols are forced to allocate fixed buffer
sizes or otherwise limit the length of a language tag. A conformant
implementation or specification MAY refuse to support the storage of
language tags that exceed a specified length. Any such limitation
SHOULD be clearly documented, and such documentation SHOULD include
what happens to longer tags (for example, whether an error value is
generated or the language tag is truncated). A protocol that allows
tags to be truncated at an arbitrary limit, without giving any
indication of what that limit is, has the potential for causing harm
by changing the meaning of tags in substantial ways.
In practice, most language tags do not require more than a few
subtags and will not approach reasonably sized buffer limitations;
see Section 4.1.
Some specifications or protocols have limits on tag length but do not
have a fixed length limitation. For example, [RFC2231] has no
explicit length limitation: the length available for the language tag
is constrained by the length of other header components (such as the
charset's name) coupled with the 76-character limit in [RFC2047].
Thus, the "limit" might be 50 or more characters, but it could
potentially be quite small.
The considerations for assigning a buffer limit are:
Implementations SHOULD NOT truncate language tags unless the
meaning of the tag is purposefully being changed, or unless the
tag does not fit into a limited buffer size specified by a
protocol for storage or transmission.
Implementations SHOULD warn the user when a tag is truncated since
truncation changes the semantic meaning of the tag.
Implementations of protocols or specifications that are space
constrained but do not have a fixed limit SHOULD use the longest
possible tag in preference to truncation.
Protocols or specifications that specify limited buffer sizes for
language tags MUST allow for language tags of up to 33 characters.
Protocols or specifications that specify limited buffer sizes for
language tags SHOULD allow for language tags of at least 42
characters.
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The following illustration shows how the 42-character recommendation
was derived. The combination of language and extended language
subtags was chosen for future compatibility. At up to 15 characters,
this combination is longer than the longest possible primary language
subtag (8 characters):
language = 3 (ISO 639-2; ISO 639-1 requires 2)
extlang1 = 4 (each subsequent subtag includes '-')
extlang2 = 4 (unlikely: needs prefix="language-extlang1")
extlang3 = 4 (extremely unlikely)
script = 5 (if not suppressed: see Section 4.1)
region = 4 (UN M.49; ISO 3166 requires 3)
variant1 = 9 (MUST have language as a prefix)
variant2 = 9 (MUST have language-variant1 as a prefix)
total = 42 characters
Figure 7: Derivation of the Limit on Tag Length
4.3.2. Truncation of Language Tags
Truncation of a language tag alters the meaning of the tag, and thus
SHOULD be avoided. However, truncation of language tags is sometimes
necessary due to limited buffer sizes. Such truncation MUST NOT
permit a subtag to be chopped off in the middle or the formation of
invalid tags (for example, one ending with the "-" character).
This means that applications or protocols that truncate tags MUST do
so by progressively removing subtags along with their preceding "-"
from the right side of the language tag until the tag is short enough
for the given buffer. If the resulting tag ends with a single-
character subtag, that subtag and its preceding "-" MUST also be
removed. For example:
Tag to truncate: zh-Latn-CN-variant1-a-extend1-x-wadegile-private1
1. zh-Latn-CN-variant1-a-extend1-x-wadegile
2. zh-Latn-CN-variant1-a-extend1
3. zh-Latn-CN-variant1
4. zh-Latn-CN
5. zh-Latn
6. zh
Figure 8: Example of Tag Truncation
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4.4. Canonicalization of Language Tags
Since a particular language tag is sometimes used by many processes,
language tags SHOULD always be created or generated in a canonical
form.
A language tag is in canonical form when:
1. The tag is well-formed according the rules in Section 2.1 and
Section 2.2.
2. Subtags of type 'Region' that have a Preferred-Value mapping in
the IANA registry (see Section 3.1) SHOULD be replaced with their
mapped value. Note: In rare cases, the mapped value will also
have a Preferred-Value.
3. Redundant or grandfathered tags that have a Preferred-Value
mapping in the IANA registry (see Section 3.1) MUST be replaced
with their mapped value. These items either are deprecated
mappings created before the adoption of this document (such as
the mapping of "no-nyn" to "nn" or "i-klingon" to "tlh") or are
the result of later registrations or additions to this document
(for example, "zh-guoyu" might be mapped to a language-extlang
combination such as "zh-cmn" by some future update of this
document).
4. Other subtags that have a Preferred-Value mapping in the IANA
registry (see Section 3.1) MUST be replaced with their mapped
value. These items consist entirely of clerical corrections to
ISO 639-1 in which the deprecated subtags have been maintained
for compatibility purposes.
5. If more than one extension subtag sequence exists, the extension
sequences are ordered into case-insensitive ASCII order by
singleton subtag.
Example: The language tag "en-A-aaa-B-ccc-bbb-x-xyz" is in canonical
form, while "en-B-ccc-bbb-A-aaa-X-xyz" is well-formed but not in
canonical form.
Example: The language tag "en-BU" (English as used in Burma) is not
canonical because the 'BU' subtag has a canonical mapping to 'MM'
(Myanmar), although the tag "en-BU" maintains its validity.
Canonicalization of language tags does not imply anything about the
use of upper or lowercase letters when processing or comparing
subtags (and as described in Section 2.1). All comparisons MUST be
performed in a case-insensitive manner.
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When performing canonicalization of language tags, processors MAY
regularize the case of the subtags (that is, this process is
OPTIONAL), following the case used in the registry. Note that this
corresponds to the following casing rules: uppercase all non-initial
two-letter subtags; titlecase all non-initial four-letter subtags;
lowercase everything else.
Note: Case folding of ASCII letters in certain locales, unless
carefully handled, sometimes produces non-ASCII character values.
The Unicode Character Database file "SpecialCasing.txt" defines the
specific cases that are known to cause problems with this. In
particular, the letter 'i' (U+0069) in Turkish and Azerbaijani is
uppercased to U+0130 (LATIN CAPITAL LETTER I WITH DOT ABOVE).
Implementers SHOULD specify a locale-neutral casing operation to
ensure that case folding of subtags does not produce this value,
which is illegal in language tags. For example, if one were to
uppercase the region subtag 'in' using Turkish locale rules, the
sequence U+0130 U+004E would result instead of the expected 'IN'.
Note: if the field 'Deprecated' appears in a registry record without
an accompanying 'Preferred-Value' field, then that tag or subtag is
deprecated without a replacement. Validating processors SHOULD NOT
generate tags that include these values, although the values are
canonical when they appear in a language tag.
An extension MUST define any relationships that exist between the
various subtags in the extension and thus MAY define an alternate
canonicalization scheme for the extension's subtags. Extensions MAY
define how the order of the extension's subtags are interpreted. For
example, an extension could define that its subtags are in canonical
order when the subtags are placed into ASCII order: that is,
"en-a-aaa-bbb-ccc" instead of "en-a-ccc-bbb-aaa". Another extension
might define that the order of the subtags influences their semantic
meaning (so that "en-b-ccc-bbb-aaa" has a different value from
"en-b-aaa-bbb-ccc"). However, extension specifications SHOULD be
designed so that they are tolerant of the typical processes described
in Section 3.7.
4.5. Considerations for Private Use Subtags
Private use subtags, like all other subtags, MUST conform to the
format and content constraints in the ABNF. Private use subtags have
no meaning outside the private agreement between the parties that
intend to use or exchange language tags that employ them. The same
subtags MAY be used with a different meaning under a separate private
agreement. They SHOULD NOT be used where alternatives exist and
SHOULD NOT be used in content or protocols intended for general use.
Phillips & Davis Best Current Practice [Page 45]
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Private use subtags are simply useless for information exchange
without prior arrangement. The value and semantic meaning of private
use tags and of the subtags used within such a language tag are not
defined by this document.
Subtags defined in the IANA registry as having a specific private use
meaning convey more information that a purely private use tag
prefixed by the singleton subtag 'x'. For applications, this
additional information MAY be useful.
For example, the region subtags 'AA', 'ZZ', and in the ranges
'QM'-'QZ' and 'XA'-'XZ' (derived from ISO 3166 private use codes) MAY
be used to form a language tag. A tag such as "zh-Hans-XQ" conveys a
great deal of public, interchangeable information about the language
material (that it is Chinese in the simplified Chinese script and is
suitable for some geographic region 'XQ'). While the precise
geographic region is not known outside of private agreement, the tag
conveys far more information than an opaque tag such as "x-someLang",
which contains no information about the language subtag or script
subtag outside of the private agreement.
However, in some cases content tagged with private use subtags MAY
interact with other systems in a different and possibly unsuitable
manner compared to tags that use opaque, privately defined subtags,
so the choice of the best approach sometimes depends on the
particular domain in question.
5. IANA Considerations
This section deals with the processes and requirements necessary for
IANA to undertake to maintain the subtag and extension registries as
defined by this document and in accordance with the requirements of
[RFC2434].
The impact on the IANA maintainers of the two registries defined by
this document will be a small increase in the frequency of new
entries or updates.
5.1. Language Subtag Registry
Upon adoption of this document, the registry will be initialized by a
companion document: [RFC4645]. The criteria and process for
selecting the initial set of records are described in that document.
The initial set of records represents no impact on IANA, since the
work to create it will be performed externally.
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RFC 4646 Tags for Identifying Languages September 2006
The new registry MUST be listed under "Language Tags" at
<http://www.iana.org/numbers.html>, replacing the existing
registrations defined by [RFC3066]. The existing set of registration
forms and RFC 3066 registrations MUST be relabeled as "Language Tags
(Obsolete)" and maintained (but not added to or modified).
Future work on the Language Subtag Registry SHALL be limited to
inserting or replacing whole records preformatted for IANA by the
Language Subtag Reviewer as described in Section 3.3 of this document
and archiving the forwarded registration form.
Each record MUST be sent to iana@iana.org with a subject line
indicating whether the enclosed record is an insertion of a new
record (indicated by the word "INSERT" in the subject line) or a
replacement of an existing record (indicated by the word "MODIFY" in
the subject line). Records MUST NOT be deleted from the registry.
IANA MUST place any inserted or modified records into the appropriate
section of the language subtag registry, grouping the records by
their 'Type' field. Inserted records MAY be placed anywhere in the
appropriate section; there is no guarantee of the order of the
records beyond grouping them together by 'Type'. Modified records
MUST overwrite the record they replace.
Included in any request to insert or modify records MUST be a new
File-Date record. This record MUST be placed first in the registry.
In the event that the File-Date record present in the registry has a
later date than the record being inserted or modified, the existing
record MUST be preserved.
5.2. Extensions Registry
The Language Tag Extensions Registry will also be generated and sent
to IANA as described in Section 3.7. This registry can contain at
most 35 records, and thus changes to this registry are expected to be
very infrequent.
Future work by IANA on the Language Tag Extensions Registry is
limited to two cases. First, the IESG MAY request that new records
be inserted into this registry from time to time. These requests
MUST include the record to insert in the exact format described in
Section 3.7. In addition, there MAY be occasional requests from the
maintaining authority for a specific extension to update the contact
information or URLs in the record. These requests MUST include the
complete, updated record. IANA is not responsible for validating the
information provided, only that it is properly formatted. It should
reasonably be seen to come from the maintaining authority named in
the record present in the registry.
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6. Security Considerations
Language tags used in content negotiation, like any other information
exchanged on the Internet, might be a source of concern because they
might be used to infer the nationality of the sender, and thus
identify potential targets for surveillance.
This is a special case of the general problem that anything sent is
visible to the receiving party and possibly to third parties as well.
It is useful to be aware that such concerns can exist in some cases.
The evaluation of the exact magnitude of the threat, and any possible
countermeasures, is left to each application protocol (see BCP 72
[RFC3552] for best current practice guidance on security threats and
defenses).
The language tag associated with a particular information item is of
no consequence whatsoever in determining whether that content might
contain possible homographs. The fact that a text is tagged as being
in one language or using a particular script subtag provides no
assurance whatsoever that it does not contain characters from scripts
other than the one(s) associated with or specified by that language
tag.
Since there is no limit to the number of variant, private use, and
extension subtags, and consequently no limit on the possible length
of a tag, implementations need to guard against buffer overflow
attacks. See Section 4.3 for details on language tag truncation,
which can occur as a consequence of defenses against buffer overflow.
Although the specification of valid subtags for an extension (see
Section 3.7) MUST be available over the Internet, implementations
SHOULD NOT mechanically depend on it being always accessible, to
prevent denial-of-service attacks.
7. Character Set Considerations
The syntax in this document requires that language tags use only the
characters A-Z, a-z, 0-9, and HYPHEN-MINUS, which are present in most
character sets, so the composition of language tags should not have
any character set issues.
Rendering of characters based on the content of a language tag is not
addressed in this memo. Historically, some languages have relied on
the use of specific character sets or other information in order to
infer how a specific character should be rendered (notably this
applies to language- and culture-specific variations of Han
ideographs as used in Japanese, Chinese, and Korean). When language
Phillips & Davis Best Current Practice [Page 48]
RFC 4646 Tags for Identifying Languages September 2006
tags are applied to spans of text, rendering engines sometimes use
that information in deciding which font to use in the absence of
other information, particularly where languages with distinct writing
traditions use the same characters.
8. Changes from RFC 3066
The main goals for this revision of language tags were the following:
*Compatibility.* All RFC 3066 language tags (including those in the
IANA registry) remain valid in this specification. The changes in
this document represent additional constraints on language tags.
That is, in no case is the syntax more permissive and processors
based on the ABNF and other provisions of RFC 3066 (such as those
described in [XMLSchema]) will be able to process the tags described
by this document. In addition, this document defines language tags
in such as way as to ensure future compatibility.
*Stability.* Because of changes in the past in the underlying ISO
standards, a valid RFC 3066 language tag could become invalid or have
its meaning change. This has the potential of invalidating content
that may have an extensive shelf-life. In this specification, once a
language tag is valid, it remains valid forever.
*Validity.* The structure of language tags defined by this document
makes it possible to determine if a particular tag is well-formed
without regard for the actual content or "meaning" of the tag as a
whole. This is important because the registry grows and underlying
standards change over time. In addition, it must be possible to
determine if a tag is valid (or not) for a given point in time in
order to provide reproducible, testable results. This process must
not be error-prone; otherwise implementations might give different
results. By having an authoritative registry with specific
versioning information, the validity of language tags at any point in
time can be precisely determined (instead of interpolating values
from many separate sources).
*Utility.* It is sometimes important to be able to differentiate
between written forms of a language -- for many implementations this
is more important than distinguishing between the spoken variants of
a language. Languages are written in a wide variety of different
scripts, so this document provides for the generative use of ISO
15924 script codes. Like the generative use of ISO language and
country codes in RFC 3066, this allows combinations to be produced
without resorting to the registration process. The addition of UN
M.49 codes provides for the generation of language tags with regional
scope, which is also required by some applications.
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RFC 4646 Tags for Identifying Languages September 2006
The recast of the registry from containing whole language tags to
subtags is a key part of this. An important feature of RFC 3066 was
that it allowed generative use of subtags. This allows people to
meaningfully use generated tags, without the delays in registering
whole tags or the need to register all of the combinations that might
be useful.
The choice of placing the extended language and script subtags
between the primary language and region subtags was widely debated.
This design was chosen because the prevalent matching and content
negotiation schemes rely on the subtags being arranged in order of
increasing specificity. That is, the subtags that mark a greater
barrier to mutual intelligibility appear left-most in a tag. For
example, when selecting content written in Azerbaijani, the script
(Arabic, Cyrillic, or Latin) represents a greater barrier to
understanding than any regional variations (those associated with
Azerbaijan or Iran, for example). Individuals who prefer documents
in a particular script, but can deal with the minor regional
differences, can therefore select appropriate content. Applications
that do not deal with written content will continue to omit these
subtags.
*Extensibility.* Because of the widespread use of language tags, it
is disruptive to have periodic revisions of the core specification,
even in the face of demonstrated need. The extension mechanism
provides for a way for independent RFCs to define extensions to
language tags. These extensions have a very constrained, well-
defined structure that prevents extensions from interfering with
implementations of language tags defined in this document.
The document also anticipates features of ISO 639-3 with the addition
of the extended language subtags, as well as the possibility of other
ISO 639 parts becoming useful for the formation of language tags in
the future.
The use and definition of private use tags have also been modified,
to allow people to use private use subtags to extend or modify
defined tags and to move as much information as possible out of
private use and into the regular structure.
The goal for each of these modifications is to reduce or eliminate
the need for future revisions of this document.
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RFC 4646 Tags for Identifying Languages September 2006
The specific changes in this document to meet these goals are:
o Defines the ABNF and rules for subtags so that the category of all
subtags can be determined without reference to the registry.
o Adds the concept of well-formed vs. validating processors,
defining the rules by which an implementation can claim to be one
or the other.
o Replaces the IANA language tag registry with a language subtag
registry that provides a complete list of valid subtags in the
IANA registry. This allows for robust implementation and ease of
maintenance. The language subtag registry becomes the canonical
source for forming language tags.
o Provides a process that guarantees stability of language tags, by
handling reuse of values by ISO 639, ISO 15924, and ISO 3166 in
the event that they register a previously used value for a new
purpose.
o Allows ISO 15924 script code subtags and allows them to be used
generatively. Defines a method for indicating in the registry
when script subtags are necessary for a given language tag.
o Adds the concept of a variant subtag and allows variants to be
used generatively.
o Adds the ability to use a class of UN M.49 tags for supra-national
regions and to resolve conflicts in the assignment of ISO 3166
codes.
o Defines the private use tags in ISO 639, ISO 15924, and ISO 3166
as the mechanism for creating private use language, script, and
region subtags, respectively.
o Adds a well-defined extension mechanism.
o Defines an extended language subtag, possibly for use with certain
anticipated features of ISO 639-3.
Phillips & Davis Best Current Practice [Page 51]
RFC 4646 Tags for Identifying Languages September 2006
9. References
9.1. Normative References
[ISO10646] International Organization for Standardization,
"ISO/IEC 10646:2003. Information technology --
Universal Multiple-Octet Coded Character Set (UCS)",
2003.
[ISO15924] International Organization for Standardization, "ISO
15924:2004. Information and documentation -- Codes for
the representation of names of scripts", January 2004.
[ISO3166-1] International Organization for Standardization, "ISO
3166-1:1997. Codes for the representation of names of
countries and their subdivisions -- Part 1: Country
codes", 1997.
[ISO639-1] International Organization for Standardization, "ISO
639-1:2002. Codes for the representation of names of
languages -- Part 1: Alpha-2 code", 2002.
[ISO639-2] International Organization for Standardization, "ISO
639-2:1998. Codes for the representation of names of
languages -- Part 2: Alpha-3 code, first edition",
1998.
[ISO646] International Organization for Standardization,
"ISO/IEC 646:1991, Information technology -- ISO 7-bit
coded character set for information interchange.",
1991.
[RFC2026] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2028] Hovey, R. and S. Bradner, "The Organizations Involved
in the IETF Standards Process", BCP 11, RFC 2028,
October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26,
RFC 2434, October 1998.
Phillips & Davis Best Current Practice [Page 52]
RFC 4646 Tags for Identifying Languages September 2006
[RFC2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum
of Understanding Concerning the Technical Work of the
Internet Assigned Numbers Authority", RFC 2860,
June 2000.
[RFC3339] Klyne, G., Ed. and C. Newman, "Date and Time on the
Internet: Timestamps", RFC 3339, July 2002.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[UN_M.49] Statistics Division, United Nations, "Standard Country
or Area Codes for Statistical Use", UN Standard
Country or Area Codes for Statistical Use, Revision 4
(United Nations publication, Sales No. 98.XVII.9,
June 1999.
9.2. Informative References
[RFC1766] Alvestrand, H., "Tags for the Identification of
Languages", RFC 1766, March 1995.
[RFC2047] Moore, K., "MIME (Multipurpose Internet Mail
Extensions) Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
[RFC2231] Freed, N. and K. Moore, "MIME Parameter Value and
Encoded Word Extensions: Character Sets, Languages,
and Continuations", RFC 2231, November 1997.
[RFC2781] Hoffman, P. and F. Yergeau, "UTF-16, an encoding of
ISO 10646", RFC 2781, February 2000.
[RFC3066] Alvestrand, H., "Tags for the Identification of
Languages", BCP 47, RFC 3066, January 2001.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing
RFC Text on Security Considerations", BCP 72,
RFC 3552, July 2003.
[RFC4645] Ewell, D., Ed., "Initial Language Subtag Registry",
RFC 4645, September 2006.
[RFC4647] Phillips, A., Ed. and M. Davis, Ed., "Matching of
Language Tags", BCP 47, RFC 4647, September 2006.
Phillips & Davis Best Current Practice [Page 53]
RFC 4646 Tags for Identifying Languages September 2006
[Unicode] Unicode Consortium, "The Unicode Standard, Version
5.0", Boston, MA, Addison-Wesley, 2007. ISBN 0-321-
48091-0.
[XML10] Bray (et al), T., "Extensible Markup Language (XML)
1.0", 02 2004.
[XMLSchema] Biron, P., Ed. and A. Malhotra, Ed., "XML Schema Part
2: Datatypes Second Edition", 10 2004, <
http://www.w3.org/TR/xmlschema-2/>.
[iso639.prin] ISO 639 Joint Advisory Committee, "ISO 639 Joint
Advisory Committee: Working principles for ISO 639
maintenance", March 2000, <http://www.loc.gov/
standards/iso639-2/iso639jac_n3r.html>.
[record-jar] Raymond, E., "The Art of Unix Programming", 2003,
<urn:isbn:0-13-142901-9>.
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Appendix A. Acknowledgements
Any list of contributors is bound to be incomplete; please regard the
following as only a selection from the group of people who have
contributed to make this document what it is today.
The contributors to RFC 3066 and RFC 1766, the precursors of this
document, made enormous contributions directly or indirectly to this
document and are generally responsible for the success of language
tags.
The following people (in alphabetical order) contributed to this
document or to RFCs 1766 and 3066:
Glenn Adams, Harald Tveit Alvestrand, Tim Berners-Lee, Marc Blanchet,
Nathaniel Borenstein, Karen Broome, Eric Brunner, Sean M. Burke, M.T.
Carrasco Benitez, Jeremy Carroll, John Clews, Jim Conklin, Peter
Constable, John Cowan, Mark Crispin, Dave Crocker, Elwyn Davies,
Martin Duerst, Frank Ellerman, Michael Everson, Doug Ewell, Ned
Freed, Tim Goodwin, Dirk-Willem van Gulik, Marion Gunn, Joel Halpren,
Elliotte Rusty Harold, Paul Hoffman, Scott Hollenbeck, Richard
Ishida, Olle Jarnefors, Kent Karlsson, John Klensin, Erkki
Kolehmainen, Alain LaBonte, Eric Mader, Ira McDonald, Keith Moore,
Chris Newman, Masataka Ohta, Dylan Pierce, Randy Presuhn, George
Rhoten, Felix Sasaki, Markus Scherer, Keld Jorn Simonsen, Thierry
Sourbier, Otto Stolz, Tex Texin, Andrea Vine, Rhys Weatherley, Misha
Wolf, Francois Yergeau and many, many others.
Very special thanks must go to Harald Tveit Alvestrand, who
originated RFCs 1766 and 3066, and without whom this document would
not have been possible. Special thanks must go to Michael Everson,
who has served as Language Tag Reviewer for almost the complete
period since the publication of RFC 1766. Special thanks to Doug
Ewell, for his production of the first complete subtag registry, and
his work in producing a test parser for verifying language tags.
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Appendix B. Examples of Language Tags (Informative)
Simple language subtag:
de (German)
fr (French)
ja (Japanese)
i-enochian (example of a grandfathered tag)
Language subtag plus Script subtag:
zh-Hant (Chinese written using the Traditional Chinese script)
zh-Hans (Chinese written using the Simplified Chinese script)
sr-Cyrl (Serbian written using the Cyrillic script)
sr-Latn (Serbian written using the Latin script)
Language-Script-Region:
zh-Hans-CN (Chinese written using the Simplified script as used in
mainland China)
sr-Latn-CS (Serbian written using the Latin script as used in
Serbia and Montenegro)
Language-Variant:
sl-rozaj (Resian dialect of Slovenian
sl-nedis (Nadiza dialect of Slovenian)
Language-Region-Variant:
de-CH-1901 (German as used in Switzerland using the 1901 variant
[orthography])
sl-IT-nedis (Slovenian as used in Italy, Nadiza dialect)
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RFC 4646 Tags for Identifying Languages September 2006
Language-Script-Region-Variant:
sl-Latn-IT-nedis (Nadiza dialect of Slovenian written using the
Latin script as used in Italy. Note that this tag is NOT
RECOMMENDED because subtag 'sl' has a Suppress-Script value of
'Latn')
Language-Region:
de-DE (German for Germany)
en-US (English as used in the United States)
es-419 (Spanish appropriate for the Latin America and Caribbean
region using the UN region code)
Private use subtags:
de-CH-x-phonebk
az-Arab-x-AZE-derbend
Extended language subtags (examples ONLY: extended languages MUST be
defined by revision or update to this document):
zh-min
zh-min-nan-Hant-CN
Private use registry values:
x-whatever (private use using the singleton 'x')
qaa-Qaaa-QM-x-southern (all private tags)
de-Qaaa (German, with a private script)
sr-Latn-QM (Serbian, Latin-script, private region)
sr-Qaaa-CS (Serbian, private script, for Serbia and Montenegro)
Tags that use extensions (examples ONLY: extensions MUST be defined
by revision or update to this document or by RFC):
en-US-u-islamCal
zh-CN-a-myExt-x-private
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RFC 4646 Tags for Identifying Languages September 2006
en-a-myExt-b-another
Some Invalid Tags:
de-419-DE (two region tags)
a-DE (use of a single-character subtag in primary position; note
that there are a few grandfathered tags that start with "i-" that
are valid)
ar-a-aaa-b-bbb-a-ccc (two extensions with same single-letter
prefix)
Authors' Addresses
Addison Phillips (Editor)
Yahoo! Inc.
EMail: addison@inter-locale.com
Mark Davis (Editor)
Google
EMail: mark.davis@macchiato.com or mark.davis@google.com
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
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made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
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Copies of IPR disclosures made to the IETF Secretariat and any
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA).
Phillips & Davis Best Current Practice [Page 59]