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.
When "their generation" are a collective, ( given the present tense preference of Vietnamese) do not is the way to say it. To say "they does not" is terrible English
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.