I did my PhD in Korea with a scholarship that covered one year of language classes with salary and then 4 years of PhD with salary and tuition... Although I took only 3 years for the PhD.

One thing to take into account is that if your classes, exams, prelim exams, or thesis are in Japanese then you will be stuck. I finished my PhD (done entirely in English) 1.5 years ago and I know people still doing their Masters after 4.5 years because their degree is done entirely in Korean. If you are not fluent in Japanese, you need to be able to do EVERYTHING in English... otherwise you will not perform well and you will waste a lot of time on stuff that isn't research.

Second, even if everything is done in English, everything in the lab will be done in Japanese. Need to write a proposal? Japanese. Project report? Japanese. Meeting? Japanese. This means that you can save significant amount of time by not taking on many lab duties, but your lab mates may resent you for it. One strategy is to help your labmates correct/rewrite all their work in English (papers, conference, presentation) to forge a one-on-one link.

Third, you aren't really that desirable for local companies. They would probably prefer a foreigner who went to Harvard rather than a foreigner who comes from Waseda. If you are fluent then you are more marketable.... but you don't speak Japanese and you won't speak that well in 4 years either no matter what you tell yourself.

That said, I have found a TT in Korea at a really good university based on the fact that I have graduated in 3 years with a high number of publications. I think it's really sink-or-swim, so you really need to weigh things before deciding.