That's the wrong kind of environment to learn a foreign language. Except for speaking with the teacher in class, many students almost never use Japanese at all. "You speak in Mandarin with your friends after class, you use Taiwanese with your roommate, you eat Chinese food, you play mahjong.
這樣的語言環境當然不好，不少學生除了上課時跟老師的對話外，幾乎可以不用日文，「下課跟同學說國語、回家跟哥哥、室友說台語、吃中國料理、打中國麻將… …，在東京也可以過台北生活」，一名學生形容。