After I change the language to Russian and resetting the device to its factory default settings, the language has been changed back to English (the default language) again.
Is it possible to make Russian the default language even after resetting the device to its factory default settings?
The default language is defined by the country code of the device.
If the country code of the device is USA, the default language is English.
However, the country code cannot be modified by yourself. It will be set before shipping and each country have their own country code.