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.
Multiselect lists are outdated design, requiring prior knowledge of what they are and the use of keyboard (that you might not have if using a touchscreen device) for a simple interaction with a webpage is terrible UX. The issue is that it’s how browsers render them by default, a ‘proper’ multiselect requires a bunch of css and javascript.