Hi Eric,
you're right, the intention is that the preferred language of the recipient is used for these messages. This currently doesn't happen and that's a bug, probably a rather old one. I've created a tracker entry (CONTRIB-5182). Would you mind joining in there and letting me know which Scheduler version you use? It's probably not too hard to fix, but having someone to test the fix is always good.
By the way, if you (or others) have translations of the Scheduler language strings, you can contribute them as http://lang.moodle.org/ for the benefit of other users.