do you really need to use the Japanese language? i mean, by your typing. it sounds like you understand english? Also the osu!osx is still in beta/development so there are tons of bugs, i would email this to peppy for a fix if i were you
EDIT: Just re-red the post, i dun f*ked up, there should be a setting in the options menu that makes foreign character english. Dunno if this or the other one is right, so just correct me.