Add missing (though unused in regular UH) LANGUAGENAMES getter

This will probably only see use in credits-related code as well.
Some more-or-less hackish/slow translation author extraction

This does rely on manual syncs every now and then (ideally not only
shortly before releases, but definitely once before) and tagging the
commit that did update last with `weblate-credits`.

Closes #2205
Update translation AUTHORS

Automated: `update_translations` with `since`='first-weblate-commit..'