If by ´massive immigration´ you are referring to the 3.7% (IIRC, 11 million from the South, into 300 million people) it sounds like an exaggeration of the word.  Spain is directly behind the US in the number of immigrants (about 10% of 44 million) and it´s the side of Colorado.  Considering immigration only a source of division really reflects the old, myopic, self-sufficient-enclosure attitude, that causes its own problem:

the issues faced in attempting to unite peoples separated by culture and language continuously reinforced by national identity.

Again it seems like the US talking in the mirror, but not seeing.

First generation Americans speak wonderful English,

or multiple generation ones, depending on the quality of their education and only if you define ´Americans´ as USians and some Canadians.

often in addition to the language of their parents, and they don't want to "go back home."

Other than Americans from the South, few enclaves maintain their parents´ language and if they are first generation USians, aren´t they "at home" already?

Our knowledge has surpassed our wisdom. -Charu Saxena.