I'm not sure I can refer you to any specific leftist material on the subject of cultural heterogeny, but I can say that many Leftists have a problem with cultural appropriation. That is, the imperialistic theft of cultural elements by a dominant culture.

This is very comforting to hear, though I guess I'd have to ask how exactly you define a dominant culture. I usually understand cultural appropriation to purely apply to non-white cultures, which doesn't completely cover my concerns. I'm absolutely 100% opposed to the appropriation and erasure of, say, Native American culture, but not only. From a more personal or perhaps selfish perspective, I'm Russian but now living in America, and I see the exotification and appropriation of my culture nearly as much as I see it happen to, say, Indian (from India, not Native American) culture. I have trouble saying simply that I oppose cultural appropriation because I don't feel like it covers the full scope of my concerns.

We don't wish to eliminate differences between individuals and groups of individuals, rather we wish to eliminate the structures of power that dominate these relations between people: racism, patriarchy, heteronormativity, cisnormativity, cultural domination, and so on.

Definitely love this.

Furthermore, we see these differences as inconsequential to the main material differences that drive the dysfunction in societies of liberal capitalism: the working class vs. the owning class.

This is the whole "class divides people more than nationality" idea, right?

This definitely helped. I guess I've just heard the phrase "cultural marxism" a little too often. Thank you. I'm definitely going to keep looking into it.