What is the “Language” field for and why am I asked to enter it for every entity, including authors and publishers?
That’s the language of the entered name, as far as I understand (but that needs to be made more clear). BookBrainz uses aliases as names, rather than having a primary name, IIRC, so it’s the equivalent of the alias languages in MusicBrainz.
Ah, OK I see now. When I first added a creator I thought it meant the language that they wrote in. Having the first option be [Multiple languages] is confusing (IIRC this isn’t an option for MB aliases). Also, I think it would be better to make this field optional for submission.