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 (C) The Internet Society (2005).
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.
Table of Contents
1. Introduction ....................................................32. The Language Tag ................................................42.1. Syntax .....................................................42.2. Language Subtag Sources and Interpretation .................72.2.1. Primary Language Subtag .............................82.2.2. Extended Language Subtags ..........................102.2.3. Script Subtag ......................................112.2.4. Region Subtag ......................................112.2.5. Variant Subtags ....................................132.2.6. Extension Subtags ..................................142.2.7. Private Use Subtags ................................162.2.8. Preexisting RFC 3066 Registrations .................162.2.9. Classes of Conformance .............................173. Registry Format and Maintenance ................................183.1. Format of the IANA Language Subtag Registry ...............183.2. Language Subtag Reviewer ..................................243.3. Maintenance of the Registry ...............................243.4. Stability of IANA Registry Entries ........................253.5. Registration Procedure for Subtags ........................293.6. Possibilities for Registration ............................323.7. Extensions and Extensions Registry ........................343.8. Initialization of the Registries ..........................374. Formation and Processing of Language Tags ......................384.1. Choice of Language Tag ....................................384.2. Meaning of the Language Tag ...............................404.3. Length Considerations .....................................414.3.1. Working with Limited Buffer Sizes ..................424.3.2. Truncation of Language Tags ........................434.4. Canonicalization of Language Tags .........................444.5. Considerations for Private Use Subtags ....................455. IANA Considerations ............................................465.1. Language Subtag Registry ..................................465.2. Extensions Registry .......................................476. Security Considerations ........................................487. Character Set Considerations ...................................488. Changes from RFC 3066 ..........................................499. References .....................................................529.1. Normative References ......................................529.2. Informative References ....................................53
Appendix A. Acknowledgements ......................................55
Appendix B. Examples of Language Tags (Informative) ...............56
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
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
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
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].
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.
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.
The syntax of the language tag in ABNF [RFC4234] is:
Language-Tag = langtag
/ privateuse ; private use tag
/ grandfathered ; grandfathered registrations
langtag = (language
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.
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
o [ISO639-1] recommends that language codes be written in lowercase
o [ISO3166-1] recommends that country codes be capitalized ('MN'
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
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
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
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
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:
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
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
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
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
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
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."
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
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'.
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
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.
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
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
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
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
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.
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
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"
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
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.
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
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
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
For example, if the prefix singleton 'r' and the shown subtags were
defined, then the following tag would be a valid example:
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
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.
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
An implementation that claims to check for well-formed language tags
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-
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