I would kind of disagree with this, there are translators (I use GoGoDuck for searches) that do a really good job. One game I play the developers native language is Russian but the game is in English. However when I open the save file in RPGEditor and go into the variables to check where I'm at in relationship points its all in Russian.
That's not what your suggesting, and trying to use a translator and convert an entire game of dialog is going to take a long time. I do agree that this person is better off trying to learn the a basic language but using a translator would go a long way to help them out, once they understand English translated to Vietnamese the wording becomes easier to understand.
And then there's always these language teaching tools, but they seem to only teach bare essentials of a language, its not a full on speak fluent (example) Spanish or Vietnamese.