That is the problem.
THEY are the ones unwilling to adapt. They are the ones who reach a new place and, instead of learning the language, the culture, the history and the traditions, try to force theirs on the locals.
When in Rome, act like a Roman, not like a freaking Celt. If you're going somewhere, YOU should adapt to THEIR customs, not the other way around. All my years living abroad, I never tried to force people around me into treating me different for having a different culture, and I never separated myself from others to enclose myself with the few ones who spoke my native language either.
Integration? How do you integrate those who do not wish to be integrated? You simply can't. A part of the problem will eventually work itself out after some generations because of mixed marriages and such. But the only true way to integrate people into a culture they want no part of, is simply by violence.