Fwiw last I checked (and this may have been fixed since, haven’t reviewed), the Lemmy software was bad about not providing any feedback when communities set their languages, meaning if people tried to post or comment with their defaults as-is (which back then was Undetermined as I recall), they simply couldn’t and Lemmy wouldn’t indicate why.

Due to this lack of feedback, I think many communities opted for undetermined to avoid confusing people by appearing broken. If I’m mistaken and/or this has since been adjusted to provide better feedback, let me know!