Well, I'm a little late and the topic is too huge to be analyzed in the size of a reddit post but here's my take. I think that radical leftism is both compatible and incompatible with what you're saying.

It's compatible in the sense that none of us seek to destroy or erase the differences in culture that exist throughout humanity. The Left is (or at least should be) about reclaiming the concepts of group identity/culture/homeland/people from the Right and reshaping them into a healthy state that can be widely celebrated. The differences between us are only natural and cannot be erased even if you try to do it. Different groups have their natural particularities just like a tree's branches differ in size, position on the tree, shape and so on. It's important to acknowledge that, and it's equally important to acknowledge that even though we are different "branches", we're all organic parts of one "tree". Internationalism, solidarity, cooperation, peace, all that are not meant to erase cultures and create an amorphous blob. They are meant to enhance the realization of our common human condition and the bonds between us. There's no need to destroy everything and try to create one "global culture", not only because that's unnecessary but also because it's impossible, our diversity as individuals and groups is a fact. I don't see how radical leftism would ever oppose that, historically it hasn't at least and I hope we stay on the same path. Trying to erase our diversity would be fighting windmills. What is needed though is a common understanding, a common foundation, a common set of values, and here is where I think there's an incompatibility with what you're saying.

I don't support the preservation of all cultures or all customs/traditions (if I understood correctly that's what you're saying) and the reason is because many of them are toxic or contain toxic elements. I doubt there can be room in the future for a culture that is massively aggressive and racist like Nazi Germany for example. I doubt there can be a coexistence or common respect with people that have the values of radical religious extremists like ISIS. For me there is no reason to respect or preserve those cultures but to oppose them, it would be impossible for me to extend a hand of friendship and coexistence to the Islamic State even if they agreed to stay in their lands, thus this is incompatible with the "everyone stays on their land, everyone has their culture, all cultures should be preserved" you're saying. I disagree with that. What we need in order to let our diversity flourish naturally is primarily to achieve a common framework in which we'll all work in. By that I mean the common basic understanding I mentioned in my previous paragraph. To give you a crude example of that, you could say that European cultures were overtaken by the common values of the Enlightenment, they had this basic understanding: humanism, secularism, a tendency to create free-er societies, and so on. At the same time that didn't stop European nations from developing individual identities, traditions, customs, etc. And that's exactly what I mean, we need a global "Enlightenment", a basic framework in which we'll all operate in. And only after we have that framework we can find common ground and coexist. If you take a Polish guy and a Greek guy for example, the fact that one of them grew up eating a different traditional food during Easter won't be a fundamental problem for them. They'll share this, they'll exchange experiences, this difference is gonna be a difference bringing them together than pulling them apart. On the other hand if you'll take an ISIS nutjob and an average secular European you'll observe that there can be any coexistence or understanding. Their differences will be chaotic and they'll lead to conflicts, hostility, and so on. What's the difference between those two cases? In one case diversity/differences act as a positive factor, a cultural social lubricant if you will. In the other case the differences act as barriers between the two, as reason for conflict and opposition. That's why I'm not for "multiculturalism" of any kind or the (mostly right wing) concept of ethnopluralism. I don't believe everyone can get along, it's not just a matter of "tolerance" and civil behavior, sometimes (many times) the differences are unbridgeable. That's why I cannot consent to the preservation of everything and everyone as it is. I believe in the active, radical, conscious construction of a common understanding first, a "Global Enlightenment" if you will, and then the flourishing of our cultures on top of this structure.