Backend/i18n: Language list contains invalid and deprecated ISO639-3 codes

[tristanlabelle]

Specifically, _tw is invented (nan is official), smi is a part2 code (group of languages), and ajp is deprecated.

[aapeliv]

Solution is not to remove them. _tw is there in addition to nan on purpsoe

[tristanlabelle]

Further thinking / context: #9204 (comment)