If language isn't the thing that matters for national harmony, then this is at best a red herring and at worst a fig leaf.
Culture is language + religion + food + social norms + ideology and probably other things. Not everything needs to be the same, but if too many are in conflict, social problems arise.
I feel like the more this is being explained to me the more unsavory it seems.
But evidence of this is all around the world. Hindu mobs attacks butchers who sell beef in India, there's tons of contention over the rise of halal butcher shops and Muslim demands for halal options in the west, or even vegans that protest butcher shops for a non-religious example.
For religion itself, tons of wars have been started over it, that's easy.
As for ideology, look at the US and how fractured it is because of politics today. Or the historic communist revolutions. Or political violence in South and Central America. Both left-wing and right-wing violence against the other side are a thing.
People shouldn't care about these things, but they do, it's a thing.
Not getting it. Starting to lean towards the "fig leaf" theory.
But it's pretty obvious that tribalism isn't dead. And what makes a tribe? And of course, we're trying to kill tribalism by simply making a more inclusive, bigger tribe. So what are the commonalities we'll have as part of that bigger tribe?
When borders don't exist anymore, racial discrimination doesn't exist, religion is dead, it won't be because we simply killed them. It'll be because we created a new ideology to supercede it.
The OP on this thread said they would be irritated by (and I'm paraphrasing) someone coming in who spoke a different language. I am aware that such a thing exists, but wanted an answer to why. What you're addressing is the unasked first question, does such hostility exist?
I got confused because it appeared earlier in the thread that you were trying to justify that viewpoint on its own consequence. But now it seems like you were just steelmanning, which honestly isn't that useful to me on questions that are basically "how does your worldview work?".
No historian worth their salt would be foolish enough to compare the circumstances of individual European countries with that of their former colonial subjects, nor things like the shifting borders and variations of thousands of years of Chinese history. Strife in China and India isn’t even independent from the influence of “homogenous“ countries in Europe!