There are some universities with courses/subjects totally in English...
Those teaching positions are in demand..

I would recommend contacting universities directly and maybe employment agencies as well (often links for them around this page).

As for teaching in Japanese.. as you would have read on this forum already, they wont employ foreigners to do jobs that Japanese can do as well or better (like teaching in Japanese!).
It would also take many many years to be proficient enough to teach in Japanese