By default the login server creates the login screen Language menu containing a list of all locales installed on the system. When the user selects a locale from the login screen language list, the login server will redisplay the login screen in the selected locale. When the user subsequently logs in, the login server will start a desktop session for the user in that locale.
You can specify your own list of languages by modifying the Dtlogin*languageList resource in Xconfig:
Dtlogin*languageList: En_US De_DE
The login server now displays only En_US and De_DE in the login screen Language menu.