You can create a User level setting for manager_language which will override the system setting with the same key when that user is logged in.
To do this, go to Manage Users > Settings tab then create a new setting with key manager_language, and value of the language ISO code you need (eg. 'en', 'fr', 'de'). You could also write a plugin to have the setting created when a new user is created.
I'm not sure if this is the recommended way but it certainly works well for me