Generally speaking, you are required to have certain Japanese language proficiency to enter Japanese universities, especially for undergraduate program.
*For information regarding courses provided in English (undergraduate programs) : http://www.u-tokyo.ac.jp/en/prospective-students/undergraduate_english.html
Center for Japanese language Education of the University of Tokyo provides Japanese language course for the students of the University of Tokyo, but there is no course in which you can enter the University of Tokyo only to study Japanese. It is advisable to learn Japanese before coming to Japan; however, those who aim to learn Japanese intensively in Japan can go to Japanese language schools or preparatory Japanese language programs at private universities, which is called "Ryugakusei-bekka".
You can gain information on Japanese language institutes from the following websites. Be sure to make inquiries directly to the institutes to gain specific information.
http://www.nisshinkyo.org/（Association for the Promotion of Japanese Language Education）
The list of preparatory Japanese language programs at private universities / junior colleges (Ryugakusei Bekka) is updated on JASSO website:http://www.jasso.go.jp/study_j/schinfo_e.html