This is not a lesson for Germany alone. In countries around the world, and over the centuries, peoples with jarring differences in language, cultures and values have been a major problem and, too often, sources of major disasters for the societies in which they co-exist.
Even the tragedies and atrocities associated with racial differences in racist countries have been exceeded by the tragedies and atrocities among people with clashing cultures who are physically indistinguishable from one another, as in the Balkans or Rwanda.