I've installed Foxmail on my computer, but I believe it's in Chinese. I couldn't find a way to start the software without adding an account. However, the simplest way to test if the software supports Vietnamese is with the help of the input tools found in Windows. Go to Settings > Regional and add the Vietnamese language. Use Alt + Shift to change between languages, then make Foxmail's windows active and compose a mail. Type the characters you need in the language you want. It should work this way.
This is how it looks in Windows 10:
