In SoCal, I don't know any second generation Mexican-Americans who don't speak English & who aren't very assimilated. They assimilate to the point where they begin to date/marry those of different ethnicities who are also American.
I'm sure those who don't assimilate more than one generation in exist, but as working/lower middle class person, I don't come across them. Those who don't speak any English & who have not assimilated much are the new immigrants, and that doesn't seem to be unusual with any group that's immigrated in any time period.
Assimilation seems to be blocked mostly by education, economic class, and legal status. Basically, those people don't have the means or motivation to assimilate.