Well, you raised an interesting question. You may find examples to support the observation you made from your own perspective. I am not sure if one should go that far to make a generalization based on that.

A co-worker of mine, a very nice Polish fellow who came to America long time ago, once aksed how my kid is doing at school. And I told him how good my kid is in school. He smiled and replied, "A normal child." Looking at my puzzled facial expression, he added, "It seems that the children of the first generation immigrants always excel in their school work, and quite outstanding in their generation. When it comes to the next generation, they performance falls to the average level." He then commented that he think the first generation immigrants bring the best part of their original culture to this new land, which helps their kids to excel. He thinks that for the next generation, that advantage fades out because of the next generation has merged better into the local culture.

This is an observation from his experience and his perspective, which I would not try to convince people here to believe. I believe that 1st genertaion immigrants from many other nations also tried to keep their original culure and language. When it comes to the next generation, it beomes harder and harder to do that. As a result, ... kid become average.

This is not a problem of language per se. It is an issue of diversity. Only a true diversified environment can promote the growth of the best.