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.
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
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].
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.
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.
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.
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:
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
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."
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'.
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.
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.
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.
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.
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.
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.
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
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.