The problem with OP’s claim is that it lacks any philosophical or historical rigor. Nation states, in their framing, people relating to themselves in terms of national identity, is a modern construction. And even within this context, you have widely different forms and articulations.

What is clear from the history of the 20th century is that appeals to nationalism and uniformity of culture, broadly constructed, drive more violence and conflict than they prevent. Organizing around their ideology led to two world wars, bloodshed and destruction at a previously unforeseen scale.

Arguably, the postwar peace was only restored by a return to an older form of sovereignty: that as semi-autonomous subjects of a multicultural empire (in this case that of the US and USSR). These have long standing historical precedents for their durability (e.g. the Roman and Ottoman empires).

If the only way we have peace is through brutal dictatorships or superpowers' hegemony, does it really invalidate what I'm saying?

Also, all those powers also attempted to spread their culture, language and religion. Hence Latin based languages, Arabic being spread as far as it has, Russian being spoke all over the former USSR, English being the new lingua franca.

Yes. Your argument that was “monocultural” societies are somehow more peaceful, and now more so than imperial rule, when the history of the 20th century demonstrates the opposite. Nationalism led to massive internal violence and attempts at creating new empires that ultimately led to subjugation of these states between two outside empires.

We're not exactly a post-national society or world yet.

So basically you're arguing in favor of imperialism? Imperialism replaces internal violence with violence (or the threat of it) from an outside oppressor. How is this a good solution?

No, I’m stridently anti-imperialist, but simply observing the historical reality that, counter to OP’s claims, “monocultural” nationalism is a modern, unstable construction, that tends toward violence.

>but simply observing the historical reality that, counter to OP’s claims, “monocultural” nationalism is a modern, unstable construction, that tends toward violence.

You state this as if it's a fact, but it simply isn't. There's lots of monocultural nations that don't have any violence problems whatsoever: Scandinavian nations come to mind immediately.

Moreover, the "golden" days of the Cold War were pretty awful for a lot of people. The US and USSR didn't go to war directly, but instead conducted a bunch of proxy wars: Afghanistan, Vietnam, Korea, etc. The 20th century after WWII was full of violence, it was just kept away from the two imperial powers mostly. Instead they instigated violence in other places to pursue their "interests", such as when the USA overthrew the democratically-elected government in Iran and installed a brutal dictator.

As for stability, I would argue that stability is something we've never had anywhere, not long-term. I don't think you can point to anything that's been continuously stable for a long time, so I would argue that instability is the norm. Trying to keep a society stable is like trying to balance on a beach ball: you can do it for a little while, but eventually you're going to fall.

The problem is that any country, if you wait long enough, has endless social problems, unrest, civil wars, and then they break up. So it's hard to find an exception.

Singapore has 4 languages and associated faiths and works pretty well.

Singapore had to import Nepalese Gurkhas to keep the peace, both because they're extremely militarily effective and also because they were seen as impartial by the different ethnic groups.