Its complicated - and tbh, very counterintuitive for a community - but the basic consequences are:
a) If you use Undetermined, you can only see Undetermined posts b) if you use English, you can see English and Undetermined posts
So the ideal is that everyone should use Undetermined BUT since that’s not mandated or well coordinated, its more practical to pick a language and then make all your posts in Undetermined. That way you’re reaching the most people possible.
The thing that I don’t get is certain communities - such a this for example - only allow posts to be made if they select a specific language, such as English in this case. It most likely has something to do with the settings of the Community itself, but I can’t help on that front.
I mean I wouldn't consider it a bad thing if only english language posts are allowed here but if this hinders people who didn't select english as their own language from posting, then I don't want that. Ideally you could select more than one language on your profile if you're multilingual or be able to change the language setting of your posts manually to match the community.