Cramming to learn Japanese on such a short time period isn't really going to do you much good. Frankly, you really don't need it. You can navigate around Japan pretty good without knowing the language and there are enough Japanese who know English well enough here and there to help you out. I've never had any difficulty with the language differences while there.
You may want to bring along a Japanese phrase book just in case you need it (I never did), but definitely bring along some good maps (especially for Tokyo).