Hey there. I majored in Japanese as an undergrad and one thing I've learned is, is that for native English speakers learning Japanese is a challenging endeavor. In general the rule of thumb is, is that you need about twice the amount of class hours of to become fluent in Japanese as you would for Spanish or French. I consider myself fluent, but only after studying Japanese for 5 years, and living there for a year and a half (and studying intensly for those 1 and 1/2 years). I can read novels, magazines, read and write, but even now I have serious doubts as to whether I could function in a college level course conducted in Japanese, much less medical school. If you are a Chinese or Korean national and speak AND read and write Korean or Chinese at a natively, you will be able to learn Japanese with much far ease than a westerner.
Unlike many other Asian countries that use latin/english terminology in medicine Japan have their own specialized vocabulary. Once again, if you know Chinese it might be a little easier to learn this vocabulary as the words have Chinese origins. That all being said, I do know 1 or 2 people extremely talented at languages who learned Japanese to a near native level within 3 years, however they were fluent in (I kid you not) over 8 other languages though.
I hate to discourage any person who is self determined but if you are currently an undergrad student with little or no previous experience with Japanese your chances are pretty slim for passing their entrance exams which are conducted soley in Japanese, and for which most locals spend months or even years preparing for. Many of those sites in English at those Japanese medical schools are there simply because they allow international students to come in for a short term clinical rotations, or have an exchange program with another institution. And Japanese have anxieties about foreigners and I think most would strongly prefer a local doctor as opposed to a foreign looking one with a foreign accent.
My post is getting rather long, so maybe I should stop here, but if you are serious about becoming a doctor in Japan, I would look into options in the US military or perhaps being a physician with the foreign service. None of these options gaurantee you placement in Japan though.
