As MMM indicated, one does NOT need a teaching certificate to teach English in Japan especially if all you are hoping to do is get picked up by one of the McEikaiwa chain schools.
And, on top of that, if one were looking to get a
TEFL certificate or equivalent, I think it would be more efficient and cost-effective to do it in your home country.
If you have a college degree with a Japanese major, why not apply for a position on the
JET Programme? Since you have Japanese-language ability, instead of the usual ALT (Assistant Language Teacher) positions, you might consider trying out for
CIR (Coordinator for International Relations).