Granted, the warning says clearly “If you deselect Undetermined, you will not see most content”, which places the onus on the individual user, as apposed to logic built into Lemmy on which posts/comments can be viewed, while also leaving it to each user to have to weed out alternate languages for their views which are incorrectly set. This is indeed leading to many posts simply not appearing (as designed and intended), which is less than ideal when trying to grow a user base/community.
If you’re on desktop you can hold ctrl and make multiple language selections. I have undetermined and English selected.
As to why this is even a thing that needs to be set I have no idea either.
The intent is to label content by language so that people can easily filter out languages they can’t read. The presumption that English, say, is the default language because the big websites are all US based doesn’t really hold in the Fediverse, where a lot of users, historically, were from Europe.
But the feature’s just kind of… broken right now, though.