I believe that if you are not a native Japanese speaker, it is very very difficult to teach Japanese in Japan. However, it is possible to teach English in Japan. There are two types of English teacher in Japan. One, you are recruited by private company/institute. The other is recruited by the government, as a Language teacher in public school because most of them are from America, most of the people will name them as ALT, but it's not limited to American.
Of course, there is another option that becoming a lecturer in the university. This required a lot of teaching experience, and most of them request a Master or PhD degree.
Here is one of the teachers, who wrote down his experience.
People give positive and negative comments about being a foreign-language teacher in Japan. I mentioned three types of language teachers here, each of them has a very different daily routine. As a private language teacher, you might need to teach at night, during the weekends. It depends on the student and institute schedule. As an ALT, it's kind of like a public school teacher but you get a little more freedom. You have to eat school meals with students, play with them in English. Although it depends on the school, students might be very shy.
Feel free to ask any questions, I could introduce them to answer your question.
Last updated Nov 19 '17 at 19:39