My experience here in NW Arkansas has been that the Mexican immigrants really have no incentive to learn English. The people who wish to do business with them hire Spanish speaking employees and cater to them.
Salvadorans, on the other hand, tend to assimilate very rapidly. They learn English, teach their kids English, become eager participants in the American way of life. They know they will never go back to El Salvador. A majority of Mexicans, however, intend to make lots of money and return to Mexico.
A lot of the Latinos here in are illiterate in Spanish; the educated ones learn English and assimilate while the others never will. They can live here as though they were south of the border, with television, radio, co-workers and neighbors all speaking Spanish.
I don’t think any of them (regardless of country of origin) intend to go back; they spend too much money on beer to be sending much home. All have the latest electronic gadgets, clothes that cost more than many Americans’; they won’t give that up.