"Undetermined" language seems to prevent seeing content in other languages.
As a noob this is likely me, but I tried to start a community on another server and I can't see the post (my other account used to make said community) made and set as english. Unless it's just me being a noob, that can't be good. A cursory look seems to confirm that I can't see anything aside from "Undetermined".
You should should choose both "undetermined" and all the the languages you want to see. From what I understand, "undetermined" makes it display posts for which the poster didn't specify a language.
I don't have any other languages to select on this instance besides Undetermined either. Seems like a configuration issue specific to startrek.website?
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.
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.
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?
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.