I wouldn't worry about language difficulties at all, especially after two years of study. Aside from reading and writing, I'd say it's a relatively easy language to pick up. My advice would be to concentrate on reading before you go, and leave the rest until you get there. I think kanji is the most lanbour intensive aspect of Japanese, so getting it down while you are relatively free is a good idea. You only need 2000 kanji to read a newspaper so get cracking. If you can recognize the kanji, then writing using a computer etc. is a piece of cake. After two years in Japan you'll be better at Japanese than the locals, if you learn to read now.
As an aside, if you learn to speak Japanese well, then there's no reason to limit yourself to teaching English. I'm sure you'll learn more about Japan working in a Japanese speaking environment.