Chances of becoming school teacher?

Hello.

I am studying at a University in Germany to become a teacher for junior high schools (subjects: English + German).

My fiancée is Japanese. If we move to Japan later, will there be any chances for me to work as a teacher in a Japanese school?

You failed ro mention you japanese language ability. Written and orally.

if you means "in public schools or other schools, authorized by the government", you can't.
but, you can be "a foreign assistant".
the difference between a foreign assistant and a teacher is that the latter has national (Japanese governmental) license for education.
once you (license holder) are hired as public school teacher, its position is firm and almost no possibility to lose the job. but, an assistant is assistant. its position is extremely unstable. (no need to hire the same person for a long time.)
generally speaking, if you (including me) want to be a teacher in junior high school, you need to graduate 教育学部, department of education, (or some other departments) in 4 year university in Japan. in that sense, teaching license is extremely local.
ask your fiancee how 教育学部 works.

of course, if you don't mind you are a teacher of unauthorized organizations, like international schools. you can be, because the government does not care their education. also, they don't care your hiring. you will be protected only by labor laws, as same as other workers in various companies. (i.e. international schools are treated as similar to company.)