Hi there...
this is my first post on this site. Im not sure if anyone will read this but I thought Id answer your question.
Japans market for english teachers is mainly in the conversation department. They will hire just about anyone (native speaker) with a degree.
A lot of schools have their own books, and teachers are trained to use these.
Basically being a conversation teacher in Japan just requires us to facilitate conversation (to put it basically), and doesnt really require any certification. Although I will say that just being a native speaker doesnt make you a good conversation teacher...
hope this was of interest to someone...

taka