A lot of this is about citizenship laws and language laws. These countries are more successful when they.... You could sort of line it up. There are all sorts of things in play here. You're looking at the extent to which populations are intermingled as well as the policies and you can't ignore that, so you don't have perfectly controlled experiments.
I would argue that policies that allow for multiple languages, for citizenship and access, are going to be more effective because they make the older generation more comfortable. The younger generation is going to learn the local language anyway but it's a way of making their parents more comfortable as well, and that eases the transition. That's the sort of thing I'm talking about.