My grandparents on my mother’s side were German.
My mother couldn’t speak English when she started school and picked up English almost immediately simply because she had to. Once my mother started school my grandmother wouldn’t pay attention to her unless she spoke English.
When I was real young, 3 maybe 4, I could understand German but I couldn’t speak it.
Same in my family, only it was Czech. I only heard my grandparents speak it to each other, and even then, only when they were arguing about something that I assumed was private. Growing up, my mother was a stickler for us using nearly perfect English grammar. She corrected us until we saw it her way. No laziness allowed!
As an aside, we recently had some renovation work done in our kitchen. Two of the workers were legal Mexicans. (Yes, I asked our general contractor.) One of them had only been here for 7 years, and he spoke beautiful English. I was surprised at his command of our language. So I asked him how he learned, and he said that his former girlfriend spoke English only, and if he wanted to speak to her, that English was all she understood. She didn’t attempt to learn Spanish. He proudly told me that he is preparing to become a US citizen as soon as he can.
Then I asked him about the other worker, who spoke English only a little. Very basic words (please, thank you, bathroom, etc.) He told me that Nicky has been here for 10 years, but lives around other Mexicans, so has little need to learn the language. He just gets by, and most people let him use Spanish.