You're right that it's complex, Janie. On the subject of people who speak only Spanish in the southwestern US, that problem is made even more complex by the fact that there are families who have lived right where they are in places like Texas and New Mexico for hundreds of years, and have only spoken Spanish. In their cases, they weren't the ones who moved, it was the borders between the US and Mexico that moved.

I have a friend whose family has lived in New Mexico for more than four hundred years, and some of them have never learned to speak English.