What jobs can you get on WHV and how important is Japanese?

I'm hoping to head from Australia to Japan sometime next year on a WHV, currently I bartend which has been fine for travelling around Australia as I speak fluent English. However, I have never learned even a little bit of Japanese so I'm currently under the impression that bartending in Japan won't be feasible. What kinds of jobs are possible for the WHV? Is it possible to learn a decent amount of Japanese for next summer?