you can filter by language.

But people aren’t setting a language on the post, so its “undefined” meaning it wouldn’t get filtered

The only solution I’m aware of at the moment is to block specific communities. You can’t block instances so, if you’re getting several communities from a French instance in your feed, you have to block each of them individually.

If you’re attempting to filter a specific community that has both English and French posts, I’m not aware of anything that does that yet.