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.

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[–] [email protected] 4 points 2 years ago* (last edited 2 years ago)

Posts in Lemmy/Kbin can have a language tag - if you select anything in that box (by default nothing is selected so it shows everything) it will only show you posts of those selected languages.
So for you https://sopuli.xyz/c/suomi is basically empty because everything is tagged as Suomi (Finnish).

Undetermined is needed because tagging is not mandatory, so a lot of posts don't have a language tag at all.