Use track-level (MusicBrainz "work") language

Tracks which are set to a language of 'no linguistic content' or 'multiple languages' in Musicbrainz have their language field set to 'eng' by beets.

I’m not sure if it matters, but I have my import languages set to: en_US en

Obviously these should be left at their correct 'zxx' or 'mul' values in the beets database.