As far as I know, all you truly need is a Bachelor's Degree to apply for most jobs since you need a Visa and to get a Visa, you need a Bachelor's. Since you are actually interested in teaching in Japan, maybe you should just major in something like International Studies or take courses that focus on teaching English as a second language.
Since you're interested in straight up teaching, and not assisting, you could always check out requirements from English teaching position ads. Gaijin pot is a well-known one. Seems that most teaching positions require you to have at least one to two years of experience and conversational Japanese. An ALT, usually doesn't require any Japanese language skills, though it wouldn't hurt to have some.
Jobs in Japan: Jobs - Japan Jobs - Japan News - Japan Info - Japan Apartments - Japan Classifieds - Japan Forums
