As the majority of the courses in the traditional undergraduate programs are taught in Japanese, a high level of proficiency in Japanese is required upon enrollment.
https://www.u-tokyo.ac.jp/ja/admissions/index.html
https://www.u-tokyo.ac.jp/en/prospective-students/undergraduate_admissions.html
For details, please see https://www.u-tokyo.ac.jp/en/prospective-students/undergraduate_admissions.html