HTTP: The Definitive Guide

G.1 First Subtag Rules

If the first subtag is:

                Two characters long, it's a language code from the ISO 639^[1]and 639-1 standards

^[1] See ISO standard 639, "Codes for the representation of names of languages."

                Three characters long, it's a language code listed in the ISO 639-2^[2] standard

^[2] See ISO 639-2, "Codes for the representation of names of languagesPart 2: Alpha-3 code."

                The letter "i," the language tag is explicitly IANA-registered

                The letter "x," the language tag is a private, nonstandard, extension subtag

The ISO 639 and 639-2 names are summarized in Table G-2.