It doesn't even take a concentration of a group of a specific origin. Take Erasmus students, for example, it's terrible. They come from various countries, so they do not speak the same mother language and do not share an identical culture; but they'll always stay together, always speak in English and not in the local language, and almost never mix with locals students or others locals (except the local drug dealer of course).

I was an Erasmus student, and some of what you describe definitely happened. A combination of factors like all the local students arrived before semester started and got the nice dorms, so all the foreigners were together in the old dorm without kitchens. All the local students had social groups and weren't looking for new friends the way other exchange students are (none of us were first years). We spoke French among ourselves because not everyone spoke English, but when we first arrived socialising in French was difficult, especially in groups, and other foreigners are much more patient and forgiving of that. I actually ended up making friends with a bunch of locals by joining a soccer team, but even there half the players were other foreigners. I wouldn't describe it as terrible at all, but everyone has an opinion.

Former Erasmus student here and I want to say that it's brilliant. Sure, you don't really end up living like a local, but you get to know people from all over Europe and the rest of the world. That's a great thing in and of itself. No need for locals. Sure, if you get to know locals that's great as well, but not doing so is not in any way "terrible".

Having only experienced it from the outside (observing perfectly functional flocks if "Erasmusses" not making local contact here, seeing friends join such flocks abroad) I even feel tempted to say "you learn a lot about all kinds of countries, except for the one you go to". The intensity of socialising inside those groups is staggering and part of the lack of contact with the local population might well be that those just can't compete in being as interesting: answering "here" to the conversation starter "where are you from" instantly makes you the least interesting person in the room. Kind of like the backpacker hostel vibe.