With the en part of Duesentrieb's CheckUsage down since forever, I hacked a quick workaround script using query.php. Example:
http://tools.wikimedia.de/~magnus/checkusagetoo.php?title=Image:Flag%20of%20...
Maybe that could be transcluded into the *real* CheckUsage tool. That aside, it does work for other language= and project= parameters. There's no interface, however; you'll have to use title=XYZ.
Magnus