> In what sense is Japan "internationalist"? Their culture is nationalist, and more closed to foreigners compared to other countries.

That's a fair point, but we need to talk about matters of degree. What is happening today and since WWII is nothing like what happened before. Japan isn't about to invade neighbors and butcher them, and that is in part an intentional outcome of the postwar order - to rid the world of (that kind of) nationalism.

Japan's constitution even forbids having a military, a clause strongly supported for generations (unless something changed very recently).