We have stuff in the database that's only covered by ISO 639-3 (we're currently using ISO 639-2). It would be nice if we could use the proper codes for these (we could invent some, but that defeats the whole point of using standardised codes).
Some issues we would need to think about:
- There are about 7500 languages in ISO 639-3. We either need to only enable them on request (probably better, we don't need that many of them) or we need a search rather than a dropdown.
- ID3's TLAN frame specifically says ISO 639-2.
- Not all ISO 639-2 codes are valid ISO 639-3. In general, the ones which were removed have their frequency set to 0 so they're not selectable, but we are using "Artificial, Other".
See http://forums.musicbrainz.org/viewtopic.php?id=2681 for example