Comrades let's make racist Japan diverse!

They have had religious mass killings. https://www.ojp.gov/ncjrs/virtual-library/abstracts/faith-fanaticism-and-fear-aum-shinrikyo-birth-and-death-terrorist

And ethnonationalism gets dark every time you play it: consider where it led Japan in the mid 20th century.

Nobody complained much when most immigrants were white Anglos and Central Europeans. The whiter the better, so it seemed.

Well, back in the day, there was some debate about the "whiteness" of Eastern and Southern Europeans, particularly Jewish ones.

Not sure Slavs were particularly welcome either.

Actually with declining birth rates and a shrinking labor force for an aging population, Japan has been giving serious consideration to its exclusion of even other Asians.

serious reconsideration*