The comments here are also interesting. Over the centuries, European society was "multicultural" only in the sense that other nationalities living within a particular country knew and respected that they were living in that country, made up of Germans, French, whatever. They adapted and learned the language while still keeping their own heritage and ways. The Armenians in Ukraine, the Jews in Italy, etc., etc., maintained themselves and flourished, but they knew that they were in a particular country. What has happened in the late 20th c. is that migrants and former colonials poured into Europe -- and the autochtonous population of each particular country now has a problem with people from "away." In Europe, made up of non-immigrant nations, the new immigrants should and must respect and adapt to their new homes.
In the U.S.A., Canada, Mexico, South America, Australia, New Zealand -- the New World, everyone other than the aboriginal populations is an immigrant, a newcomer. Yes, the British or the French or the Spanish settled first, and did so to great detriment and destruction to the native population. But the colonists were immigrants. As each new wave of immigrants arrived, they adapted to what was established.
The U.S. "melting pot" never really melted -- St. Patrick's Day, Scandinavians in Minnesota, Italian food, Tex-Mex, the Latino culture -- still very much alive. In Canada, no one has really figured out what being "Canadian" really means -- it sure means something different in Quebec vs Manitoba. Canadian = British? For a long time it seemed so, but tell that to the Ukrainians and Mennonites who settled the Prairies. Yes, they learned English, and were forced to adapt to English ways to survive and progress. Talk to them about being told to "talk white." But many of the "others" retained and developed their ancestral culture while becoming "Canadian." In Quebec, Ukrainians are tri-lingual. People can live in two worlds, and be very patriotic Canadians or Americans while still being very strong supporters of their ancestral homelands. You try telling a Dauphin or Gardenton Manitoban, here for six generations, that his/her Ukrainian Christmas Eve traditions are not Canadian! There are degrees of assimilation. Adapting, learning the language, learning the new ways but retaining your heritage create a well-rounded individual and citizen. Forgetting your roots is detrimental to the person and the culture.
To compare European and North American multiculturalism is wrong. In the Old World, each country is its own, and anyone else living there must respect the original culture and nation, and accept that this is where they have settled into. In the New World, there must be respect for the established culture, but it is not as set as in the Old World. Two or three centuries in a new place is not millennia in the homeland.
At the same time, as citizens, each immigrant group in Canada or the U.S. is passionately patriotic, and you have rabid Ukrainian Republicans and Ukrainian Democrats, or Ukrainian NDPers or Liberals or Conservatives.
This issue is not black-and-white, but the Germany's Chancellor Merkel is right.