To avoid misunderstandings: To make use of the defaulting mechanism for the regional settings, the workplace address region needs to be in place already before the user generation is being triggered initially (either manually via the corresponding action button or automatically during the migration of Employee basic data).
If the users are already generated (i.e. visible in Application and User Management workcenter), then I fully agree with Harshal, then the maintenance need to happen either centrally via Application and User Management or decentrally by each user via Personalization.
Regards,
Michael