I'd say your chances are close to slim and none to get a job teaching
English in Japan. They put a premium on "native English speakers", and any
degree or certification you might have in that area will only mean that you
will be refused with a bit more politeness than usual. 

The English language schools care about your age and country. Colleges and
universities care about "native speakers" and degrees, as well as "native
speaker" status. After that, you have a host of other occupations such as
truck driver, exotic dancer, construction worker, IT, yoga instructor, and
baseball player. 

Basically, most of the jobs require either citizenship in one of a few
English speaking countries, or actual skills in a particular profession.
Strangely enough, they don't care if you can actually teach English, so any
skills you have in that department will go unnoticed.