Japan used to not have control over Okinawa or Hokkaido. Those were separate peoples that Japan invaded and colonized, along with Korea. They would have gotten to keep Korea too if it wasn't for those damned meddling Americans and their nuclear weapons, but there are still Koreans who have been living in Japan for generations. Even though Japan themselves said "you will never be like us, you will always be lesser, now grovel and grow us our cheap rice". This is multiculturalism, you just don't recognize it because you don't know what an Ainu is.
Also, Japanese is basically permanently welded to the Chinese writing system. Japan considered removing kanji and kana in favor of romaji at one point during the Meiji era - which would have been a disaster on par with English spelling reform. But, even then, Japanese is like 10% English by weight and growing, with no actual attempt by anyone to stop this from happening.