I guess I don't have extensive experience listening to them, but if you can find anything by Happy End do it. They were most recently used in Lost In Translation soundtrack, pretty awesome folky pop stuff, with clear vocals at a decent pace. Seems like a good fit for music to learn a language to.
Coincidentally, does anyone have anything by these guys laying around? Imports, the ones that are still in production, are going for a nice chunk online-that I've found anyway.