It's interesting to observe the process of assimilation across generations as it is unfolding. I have some students in classes for beginners who have only been in the US for a few weeks, and haven't been in a school of any kind for years - if ever. Some of them come from Spanish-speaking countries but cannot read or write in their first language. There are some real challenges to teaching them, but it is rewarding to help students who need help the most. In contrast, the same students just a year later are doing academic work that some posting here would almost certainly fail.
On parent-teacher night you can see the generational process all in one place. Parents will come in, nervous about communicating in English (and often preferring to talk about their children in their first language). Sometimes the student him or herself will act as interpreter if the family uses a language the teacher doesn't speak. In some cases, the whole family will come, and a younger brother or sister who was born here in the US will act as family 'spokesman' as a native speaker of English. A process that unfolds for most families sooner or later over several generations all in one room at the same time. It's fascinating from a linguistic point of view. When a teacher really speaks none of the student's heritage language, ROTC students who are confidently bilingual patrol the school during the parent-teacher hours offering interpretation services. Often, these students just help guide the families around the school to find the rooms for specific classes/teachers. Every parent/guardian, without fail, emphasizes their urgent desire for their children to master English. None have ever expressed a concern about maintaining the culture of their country of origin.
Several nights a week I drive from one (very) urban setting to another and teach classes to adult learners. These are people like the parents of the high school students I teach during the day. They are adults with jobs and families and obligations who devote precious time and energy in learning and improving their English because they know it is as important for them as it is for their children. The waiting list to enter the class is very long.
On weekends, I travel to very different towns, and tutor the children of wealthy recent immigrant families. These students generally do not face the daunting obstacles that the students at my high school do, and they enjoy the benefits of very involved, supportive, loving families. The difference is quite telling.