it mirrors a bit the experience I had in university with foreign students/phd students. Some immigrants are only friends with immigrants from the same country and it was surprising to hear that after spending years (sometimes BSc/Msc/Phd or Work) they are completely unable to say a single word in german, have no german friends etc. Usually very unapproachable und uncomfortable to talk to.

Others with mixed groups of friends were way more approachable, I was surprised by some how well they are able to speak the language after just a few years.