Nope sadly that won't do... translations are scarce even in major ones let alone in Vietnamese... that really can't be solved just by the user him/herself either by creating it ( as in a full translation in native language requires some skillz ! ) or have the ability to understand in other universal language... I'm reluctant to mention this screen translation since I do not have experience with it but have my doubts already bout it...
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.