RFC 3066 primary subtag

RFC 3066 tags are expressed as a series of 1 or more subtags separated by hyphens.
First subtag is alphabetic, usually expressed in lower case:
- 3 letters = ISO 639-2 3-character code for a language for which no ISO 639-1 2-character code exists
- 2 letters = ISO 639-1 2-character code
- 1 letter = either i for IANA registered codes or x for unregistered private codes

Indicating Text Language, slide 3 of 5