IETF language tag

An IETF language tag is an abbreviated language code (for example, en for English, pt-BR for Brazilian Portuguese, or nan-Hant-TW for Min Nan Chinese as spoken in Taiwan using traditional Han characters) defined by the Internet Engineering Task Force (IETF) in the BCP 47 document series, which is currently composed of normative RFC 5646 (referencing the related RFC 5645) and RFC 4647, along with the normative content of the IANA Language Subtag Registry. Components of language tags are drawn from ISO 639, ISO 15924, ISO 3166-1, and UN M.49.

These language tags are used in a number of modern computing standards, including those from the IETF related to the Internet protocols such as , those from the World Wide Web Consortium such as HTML,XML and PNG, and those from other private standardization bodies such as SGML or Unicode (in some of its standard annexes), or from national or regional standard bodies like ANSI or ECMA (for example in some of their standards related to computing languages, or to bibliographic references and documents classification used in institutional libraries).

IETF language tags were first defined in RFC 1766, published in March 1995. The tags used ISO 639 two-letter language codes and ISO 3166 two-letter country codes, and allowed registration of whole tags that included variant or script subtags of three to eight letters.

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