If nationalism needs separation between peoples, which people are supposed to talk to each other?

Did you meant tribalism?
The distance between tribes should be measured not through physical measuring but by following to the book the external superficial symbolics of belonging...
Things like the clothes you ware and the spare change coffee shop chating lingo you use. Nothing of substance dont worry. Playing 007 between tribes is easy.

Globalization without killing diversity. Because slow red is pink....
Globalization to end the offshores if we want Civilization and Law...
Globalization to make every human as productive as possible not redudant and useless.

The debate of global vs national is exactly the same debate between Darwinism and Neo Darwinism. Should I compete or cooperate and why or when ? We need both things. As is now we still compete in some issues where we would be better of cooperating at a global scale....but oh well lessons take their own timing...we cant but wait...