English is the defacto language like it or not. If you want to succeed in this country you need to speak good English. You don't need a law for that, it's natural selection. The law is just another way of tellling those who's first or best language isn't English that "you aren't American, we don't like you".
I agree completely regarding your point on natural selection, but you have it backwards in your last sentence.
Wanting people to assimilate tells them "we DO like you, we want you to succeed, we don't want you isolated from the American dream and we want you to take advantage of the benefits of assimilation".
Seems to me that isolating people from society is far more hurtful.
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