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.
When you select them they should be highlighted. For me, I see them with a grey highlight at first, then when I’m selecting them (when the window is active) the highlights are blue.
You then have to click save at the bottom to apply the change.