A possible solution
I was also looking for this, for the same purpose. I'm using a custom column, language, and want to default it on new books.
I remembered there is an option to add tags when adding new books, for example, I always add a NEW tag for new books, so I can filter them for final approval with my FORMATTED column when I get around to it.
What about coming up with some language tags, and just abandon the custom column. For example, _English or something like that. Then use the same syntax for other languages so they are all nice and neat in your tag browser.
Use the tag browser or query for books in the language you are seeking.
Just a thought!