IETF language tag
An IETF BCP 47 language tag is a standardized code or tag that is used to identify human languages in the Internet. The tag structure has been standardized by the Internet Engineering Task Force (IETF) in Best Current Practice (BCP) 47; the subtags are maintained by the IANA Language Subtag Registry.
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To distinguish language variants for countries, regions, or writing systems (scripts), IETF language tags combine subtags from other standards such as ISO 639, ISO 15924, ISO 3166-1 and UN M.49. For example, the tag "en" stands for English; "es-419" for Latin American Spanish; "rm-sursilv" for Romansh Sursilvan; "sr-Cyrl" for Serbian written in Cyrillic script; "nan-Hant-TW" for Min Nan Chinese using traditional Han characters, as spoken in Taiwan; and "gsw-u-sd-chzh" for Zürich German.