This is a setting for computers with more than one UI language installed. If you enable this setting the UI language of Windows ...

This is a setting for computers with more than one UI language installed. If you enable this setting the UI language of Windows menus and dialogs language for systems with more than one language is restricted to the specific language for the selected user. If the specified language is not installed on the target computer or the policy is disabled, the language selection defaults to the language selected by the user.

To enable this policy in Windows 2000, Windows XP or Windows Server 2003, you need to use "Restrict selection of Windows menus and dialogs language" policy.