No, knowing the Japanese language is not mandatory to study in Japan. Multiple universities in Japan provide programs in English, mainly for graduate studies, which lets foreign students study even if they can't speak Japanese. Still, understanding Japanese helps with everyday activities, like getting a job while in school, and finding work after graduation. Due to this, many students take language courses while they are studying to get better at speaking Japanese. Being good at Japanese also makes it easier to fit into the culture and opens up more possibilities for internships and jobs.