Language tags syntax

Value is language tag per RFC 1766:

'fr'          ISO 639
'en-US'       ISO 639 + 3166
'no-nynorsk'  additional qualifier
'i-navajo'    IANA registry
'x-anything'  "experimental"

Case is irrelevant

Do not use period or underline as separator

RFC 1766 will be updated with ISO 639-2
(see draft-alvestrand-lang-tags-v2-00.txt)