You may prefer to celebrate Holy Mass in your own language. The Catholic Tokyo International Center has a list of churches in Tokyo and Chiba Prefecture offering Mass in German, French, Spanish, Portuguese, Polish, Korean, Mandarin Chinese, Indonesian, Vietnamese and Tagalog (click on the menu at the top of that page).
For suggestions elsewhere in Japan try www.masstimes.org. However, many of the listings on that website are incorrectly identified as English and may be in Japanese. Contact the church to check.