I see there's a bit of confusion, understandably as different servers are set up differently from each other:

That should help visualize the issue :)

No, one should select all languages one can speak including undetermined. Without the options for English or other languages one cannot see posts in that language.

The results are still the same, not having other options as a user makes any external communities lose content. Is there a way to keep the local stuff as it is, but giving users more languages to choose from?

As far as I can tell, it's recommended, since you otherwise don't see posts made in other language settings, including those that are Undetermined.

There are probably some communities that were made without a fixed language setting, or were created before it, that don't have a language marked, and as such, if you don't pick "Undetermined", it may not be possible to see those posts.

In this instance (at the moment?) the only option is undetermined. Try giving a look at https://lemm.ee/c/meta (from your own server, no need to look far) and then trying again after you have selected only undefined in your options. Pinned posts are marked as english, so those won't be visible anymore.

Or anyway that's the theory, again I'm a noob.