This is the kind of talk that scares people who live in small town settings from getting out and seeing the world. I know, because I was a small town Nebraska boy who was told the same thing from people who lived their whole lives within a 50 mile radius. Had I listened, I never would have lived up to my potential of becoming a professor at a B1G university. Parents, encourage your kids to leave and find themselves. Many will find their way right back to the place from which they came; the journey to finding home, wherever it ends up being, is very important. Unfortunately, many of us realize we need to take this journey too late in life.
I think you are gravely mistaken about "foreign grad students". At major research institutions the vast majority of classes are taught by faculty members with PhDs, not graduate students. Graduate students primarily serve in teaching assistant roles. The more important point, though, is that there are plenty of domestic faculty at all levels of education and types of institutions who don't teach well. It has nothing to do with being foreign. In fact, students should have the opportunity to listen to and learn from people who have different life experiences. I guarantee that at some point in one's collegiate experience this insight will be far greater than any textbook knowledge presented in a class.