As Japanese universities (that is, graduate schools) generally don't directly enroll students in China, they will generally let you take a course equivalent to preparatory courses for one year, which is called "graduate students" in Japan (formal master students are called "monks" or "college students" in Japan). There is no credit this year, just working under the professor and auditing the course as a transitional stage. At the end of one year, written tests and interviews are conducted according to different schools, and after passing, they become formal college students, which is what China called graduate students. Japanese universities adopt the professor decision system, so get along well with your professors! )
There are no specific language requirements. Generally speaking, you can apply for CET-4. Because it is not a liberal arts class, there is no hard and fast rule for Japanese. The exam is just the entrance exam you need to take to become a full-time college student. When applying for graduate students, you are generally required to submit past materials, topics you have done, and so on.
Tuition fees vary from school to school. State-owned is cheap, private is expensive. But there are scholarships to apply for. Generally, you can apply after formal admission, according to the regulations of each school.