I used to be able to run a search query, which I think was type:“song” AND lang:“” to show me all works that are marked as songs that have no set language; just something to do to try and clean up the DB.
Search method: Indexed search with advanced query syntax
(I couldn’t find any way to search for an unset language code, so I’m negating a regexp for languages consisting of three letters. I always have trouble getting MB’s search to work the way I’d expect, so probably there’s a better way.)