Would this be something that the site owners may consider?
Yes, but it's planned to replace the whole current system of book properties by tags, and language can be one of the tags, for sure. Of course, that info won't be retroactively present in the system for the millions of books we have registered, but you have to start somewhere!
Cool Moem. That sounds promising. Perhaps people could still be encouraged to add tags after the new system is up and running, to old listings.
Thanks Bluemchenblatt for letting me know what the OBCZ stands for! Now I can see that there isn't one in Tuebingen, where we currently are, but I did discover a small shelf-full at this Bookcrossing site http://www.bookcrossing.com/---/734914?... .
You can also look on the go hunting pages for Bücherkoffer, Bücherschrank, Bücherkasten, Bücherbox, Bücherregal, etc. These are all give/take bookshelves and boxes. I noticed in Tuebingen you even have a few Bücherbaum (book tree) listed, which also look like some kind of release zone. Most of the books will probably be in German, but I'm sure some English ones will show up too! There might be some there that are not registered with bookcrossing as these are not official zones.
You can also search for a Facebook group of expats selling used books we have one in Basel, maybe there is one near you? They probably wouldn't be registered either but it can be a good resource for affordable English books.