When i deselected "undetermined" in my settings lots of posts and comments get hidden.
Personally I try to remember to set the language when posting. Especially when not writing English.
From my experience most people are still using undetermined since it's the default choice and they don't bother changing or don't know they can change.
@sexy_peach@feddit.de If you set it to undermined then everyone can see it except other Lemmy users that have excluded it from the list of languages they're able to see