Translation:His generation does not speak Vietnamese.
Most born outside their parents’ home countries speak English only. That applies to a lot of my peers who can’t carry a long conversation in Chinese/Vietnamese. A friend of mine who has kids lament the fact, but it’s pretty much inevitable when they grow up in the US. I’m not such a great speaker in Chinese either.
It's just the American way to say it, it's not wrong per se. I commented on this a year ago but I've realized this turns up so many times in this course that it's not going to be fixed everywhere anytime soon. Now I just try to remember that collective nouns (like generation, audience, and family) take the singular verb.