For this problem, there is one idea can solve it. The problem comes from the Mi fit app itself, not come from the firmware.
If your Mi fit app is 3.4.2 (or 3.4.4.1), you can use
GadgetBridge with our
customs firmware 1.2.0.8 to flash your band to the language you want.
Then unpair your band with Mi fit app, after that, delete the Mi fit app. Download from here:
MI FIT 3.4.1(previous version). Install it and connect your band with the app. Then I think, the "Chinese ???? problem" has been solved.
If you have any problems about that, follow this thread. Thanks