Being able to speak and read japan is essential only really when youre trying to score the best deal.
Put simply: In Japan, the population reads and speaks Japanese. The best deals are therefore in Japanese.
Consider myoko for example. You can find several decent priced things out there, but if you have a fluent mate sorting out your accommodation for example or your lift tickets, chances are theyre going to find you something WAY cheaper (like the black box deal for example).
Will it diminish your time in japan? Not really. Youll still be riding happily away, you might have to figure things out a bit more or spend a bit more time researching bus routes and the like, but theres a massive network of gaijin support out there, and usually the main resorts have some kind of central focal point for the international tourists to make the situation a little easier.
Essentially youll be fine and have very little difficulties. But come expecting ZERO english and youll not be too far off the situation. Japan isnt really like Germany or France or the like where people might have a working use of English. Its best to expect none and (over)plan accordingly rather than to expect some and try and put your half baked research and plans into action.