You have to wait for the Vietnamese language. The following languages are currently available: English, Chinese, Russian, French, Spanish, Italian, German.
Installation instruction is quite easy – just download specially crafted Mi Fit APK, uninstall official Mi Fit, install the custom one and your firmware will be updated automatically! After that you can use any Mi Fit you want.
Set mobile to English and uninstall apk, install the modded apk, let it update and then change language back to Portuguese.
Hope it helps, can't help you.... :-/ I still have only Mi Band 3