I found the way to use another language for notification while locale of the phone still English.
This method doesn't need modified MiFit application or customized firmware.
Just install "Mi band tools", change the language of application to what you want.
set configuration for message style...
I tried for Thai language , if set "Mi band tools" to show notification then result is at the first picture.
if set official Mi application to show notification then result is at the second picture.
No other error .
the word I used for testing is "สวัสดีเพื่อนๆ"