I switch between three languages input methods frequently, sometimes in the same typing session. The default shortcut on a Macintosh seems to be to set keyboard shortcuts for previous/next language (I hit opt-cmd-space to go to previous language) and if you're more than bi-lingual you have to cycle until you find the one for you.

The ideal would be something like hitting fn-e for English, fn-j for Japanese and fn-g for German, but anything better than the current state would be a great improvement.