If the first subtag is:

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

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 standard

See ISO 639-2, "Codes for the representation of names of languages-Part 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.

 

Hypertext Transfer Protocol (HTTP)