Let's start with a simple thought experiment. You invite a guest into your house, give him a room, and make all your facilities available to him. You find him a job — it might be one that needs to be done, it might not — but if he runs into difficulties or loses his job you provide him with the wherewithal he requires. Eventually he brings his family over for an extended visit which turns out to be permanent and before you know it an entire part of your house has been sealed off or, as in some instances, has become a domestic no-go zone.
Your new guests refuse to participate in your home life. They deplete your budget, may not even learn your language, install a V-chip in your computer to prevent the normal range of your communications, and in the course of time begin agitating to introduce a new set of house rules which you, the proprietor, are expected to abide by. It is quite possible that you return one evening to discover that your kids have been traumatized and the house is in shambles. It may happen as you set off for work in the morning, you find your car has been torched.
Clearly, the allegory may not be exact in all its particulars, though it is on the whole a valid, if somewhat lurid, picture of what goes by the name of multiculturalism. What we call "multiculturalism" is not to be construed as identical to "diversity." The former allows the ethnocratic retention of the in-group's language, justice system, customs, and politics in semi-autonomous enclaves, as, for example, the 751 zones urbaines sensibles in France, Tower Hamlets in London and Bury Park in Luton, so-called "no drive areas" like Kreuzberg in Berlin, off-limit areas in Brussels, Amsterdam's Slotervaart district, the Rosengard quarter in Malmo, and many other high-risk neighborhoods in cities across the face of Europe, most virtual Islamic republics and many prone to communal violence; the latter assumes the orderly and beneficial mingling of different peoples in the public agora, which is the case with most immigrant communities. When we say "multiculturalism," we mean primarily a social project which approves of the voluntary segregation of many Muslim communities to pursue a life apart from, and all too often hostile to or in actual conflict with, the heritage lifestyle that has welcomed them.