The problem are with non-Europeans and with that I mean Africans, Middle East people who have a different culture and are not able to adapt or learn the language.
Are they unable to adapt and learn the language, or do they decide that it's not worth the trouble to adapt and learn the language because most of the natives will never accept them or their children or their grandchildren as "real Danes" or "real Frenchmen" or "real Russians"?

I know that in U.S. history, it was a long and painful struggle to persuade the majority of "native-born Americans" that someone born in China or Ecuador or Nigeria could become a "real" American. We're much better at this acceptance of non-European immigrants than we were in the past, but we're still working on it. However, when I read about the problems caused by non-assimilated Africans and Muslims in Europe, I always wonder: Is the real problem that they are not trying to assimilate, or that they try to assimilate but then give up because they are treated as "others"?