Yes, this was a legit issue and I was able to repro it. I just added the vi.js.erb file:
https://github.com/discourse/discourse/commit/f0653563ee5cf7376be5b236c5104661bbcd400b
@Minh_Quang_Vo can you try rebuilding your instance again?
cd /var/discourse
./launcher rebuild app
This should be fixed once the Discourse source code is updated on your server.
Also, I just noticed that there are many more Vietnamese locale files missing, as the original PR was incomplete. I will add all Vietnamese locale files later today.
Yes, that is correct! Because:
https://github.com/discourse/discourse/blob/master/.tx/config#L3