It may be possible to attend a Japanese university without being completely fluent before being admitted or passing the traditional entrance exams. Japan offers scholarships to foreign students to attend university in Japan. They don't require you to be admitted to a university in order to recieve the scholarship as placement in a university is part of the program. The language requirement for recieving the scholarship is "Applicant must be willing to learn the Japanese language and receive university education in it." If you manage to recieve the scholarship you will recieve up to a year of language instruction in Japan to prepare you to attend university.
Japanese
Government Scholarships
They don't give out very many of these but it is another avenue to consider.