Hi,
I've just tested with Vietnamese and it works. This is the translation files:
https://i.imgur.com/NalVt93.png
And this is the translated "Submit" button:
https://i.imgur.com/ltoQLDq.png
Can you check 1) the translation file name, make sure it's mb-frontend-submission-LANGCODE.mo
and mb-frontend-submission-LANGCODE.po
and 2) The translation strings?