i have a phone the language of which is English. I also have a Mi Band 3 on which I have installed Arabic package to be able to see my friends' messages in Persian (using the same characters as Arabic). If I want the Mi Band 3 to use the Arabic language characters, the language of my phone must be changed to Arabic to make the language of Mi Fit application also Arabic, and I know for sure that I don't want such a thing. How do I change the language of a single application (Mi Fit here) without rooting the phone?!
What you mention is strange. I do not have Arabic, but what I have likely applies to it.
There are at least three different places where the language (and alphabet, if different) may be relevant:
Displayed messages language/alphabet.
I have two language/alphabet combinations as input (Samsung Keyboard), but I do not have both languages for the system interface. That is fine, as the first two item are covered by completely different settings.
As for displayed messages, I found no problem in WhatsApp and Viber showing messages in a different alphabet. It is not up to any configuration of my phone. It is up to the sender to have that alphabet as an input choice.
The intelligent wrist band (mi band 3) adapts it's language with the language of the application Mi Fit with which it is paired. The Mi Fit application gets its language seeing directly from the display language of the phone. Therefore, if I set the phone language as English, the Mi Fit application and also the Mi Band 3 will be set English, and the messages in other languages (e.g., Persian) will not be shown on the band.