This policy restricts the permitted system locales to the specified list. If the list is empty, it locks the system locale ...

This policy restricts the permitted system locales to the specified list. If the list is empty, it locks the system locale to its current value. This policy does not change the existing system locale; however, the next time that an admin attempts to change the machine's system locale they will be restricted to the specified list.

The locale list is specified using language names, separated by a semi-colon (;). For example, en-US is English (United States).  Specifying "en-US;en-CA" would restrict the system locale to English (United States) and English (Canada).

If this policy is Enabled, then administrators may select a system locale only from the specified system locale list.

If this policy is Disabled or Not Configured, then administrators may select any system locale shipped with the operating system.