What majors do I need if I want to become a Japanese teacher?

You may also need some time living in Japan using the language.

You need to be Japanese and a teacher, then you are a Japanese teacher.

You mean a teacher of Japanese language in your own country? You'll need a degree in Japanese, or Asian languages & literature, or similar, then a master's in education with student teaching to get state teacher certification.

A teacher in Japan? NO. Only if you are an experienced & certified TEFL instructor, with bachelor's & preferably master's in English, Linguistics, Applied Linguistics, or similar field, teacher certification in your jurisdiction, plus TEFL/CELTA certification & experience in TEFL. For teachers at international school, they must be certified teachers in their jurisdiction & have several years of teaching experience with exceptional recommendations. Your employer must obtain your employment visa before you enter Japan. Considering the extremely high cost of living in Japan, salaries are not all that great.

Probably a degree in linguistics with a strong working knowledge of Japanese.

fluent Japanese

A teacher of the Japanese language? You need to major in Japanese. If you want to be a licensed k-12 teacher, check the requirements in your state on the state's department of education website. Few schools have programs in Japanese in elementary schools. Licensure in secondary schools often requires a major in the subject area, with an MAT in education- a one year program. To teach at the university level, you need a Ph.D.