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