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.
Either Shift + Click or Ctrl + Click, or Ctrl + Shift + Click. Pretty standard, although it’s much harder to do anything on mobile.
As for what this does, who knows. I do sometimes get errors saying I can’t comment because of language on a mobile app, however on the web page you can select the language. I have both Undetermined and English selected.