@wikiped and hom
I know this is confusing to specify feed.php for OPDS feed. I'm testing a way to redirect automatically to the feed if the query comes from an OPDS reader.
I also like to filter by language, currently I use OPDS facets with my favorite reader (Mantano). I'll add you idea to the Github issues.
I'm very busy right now with real life / work. But I have a new release almost ready for you.