This is not true. I am Polish, my phone is in Polish and Mi Band 3 is not in Polish with original 1.3.0.4 firmware. I tried the same for Italian and it is also not switched to Italian by Mi Fit. French seems to be working. So no, it's not true that these languages are supported by default. Looks...
Good job on the English firmware! I have two questions - could you do the same for Cor so it is not necessary to set phone to English so Cor can be in English with latest firmware (my GF would love that as she is on iPhone with non-English language so I was thinking I would flash it for her)...