> Multiculturalism does work. The way it works is, people with different cultures / wants / ideas form their own communities and mind their own business.
Nationalism isn't capable of "creating a third community". You can't have an association of nationalists; they can't resolve their inherently opposing interests.
Or in fact the EU itself - an association of distinct national identities. But in a sort of reverse no true Scotsman fallacy, it is popular to ascribe outwardly hostile intentions to nationalism. If a country maintains its borders and national identity, but also peacefully coexists with its neighbors (as much of Europe and indeed the world does), then it's not "true" nationalism.
nationalistic societies don’t keep to themselves. They each consider themselves “the one true society”, and generally try to expand as much as possible and make everyone else second class citizens.
By “keep to themselves” i don’t mean communities should reject outsiders who want to join. They just need to not bother outsiders who don’t support them and don’t want to join. People from completely different cultures can and do exist together, however they share some other beliefs or commonalities; people who are polar opposites generally can’t coexist unless they’re separate.
Even then, a community which rejects all outsiders and secludes itself would be fine. Like that one native tribe who attacks any modern intervention. As long as they’re not subjugating their own citizens and preventing people from leaving like North Korea.
> They each consider themselves “the one true society”, and generally try to expand as much as possible and make everyone else second class citizens.
No, that is imperialism. You're trying to eliminate the option of a society that doesn't try to expand, but also doesn't accept all outsiders, by shifting definitions.
You're describing nationalism.