Tbh you’ll probably just want to just block non-English communities that pop up in your feed. There’s no way to police what language people select when they post.

So apparently that’s why there’s this warning in user settings, “Warning: If you deselect Undetermined, you will not see most content.”, which means choosing Undetermined shows everything, and choosing otherwise is more specific.

Unless “Undetermined” is also the default for communities, meaning that English-speaking communities won’t show if you choose English if they haven’t been defined properly.

Edit: I didn’t notice comments themselves had a language setting, that seems like an excessive level of detail.