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.
ctl + click or shift + click are pretty standard for selecting multiple things, yes – but not to apply those selections. Even if I select everything on the entire list and then click save, there is no way to determine that my choices have actually been applied. The list looks exactly the same as when I first set up my account.
I’ve tried double clicking items out of curiosity, but it still looks the same.
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.
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.