1) Wipe the device, select non-default language at OOBE, complete OOBE.
2) Add user to the deice, who has a different language in his preferences.
3) After user sign-in, language will be synchronized and user session will start with user preferred language. However, lock screen will still use OOBE-selected language.

I haven't tried the configuration with two users, when language of the second user is different from owner's language (i.e. login screen UI language). I am not sure whether it has the same problem.