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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.
Just to point that this is the default and "obvious" behavior for lists since the 90's (or, rather the 80's if you count Unixes GUIs). Yet almost nobody knows it for some reason (probably because it's not obvious at all, but IDK).
That and I think in many cases people opt for the ol' box select when trying to select multiple options in a GUI, even if it's in list view.