The command localeconfig allows the user to select the locale of his system. A 'locale' is a set of conventions linked to the user's language, his environment and/or his cultural conventions (like monetary symbols, date display, ...etc.). This locale is then applied to the user's working environment. By default, a Linux system is configured with the "en_US" locale, so other locales have to be specified manually ("en_GB" for the United Kingdom, "es_ES" for Spain, and so on).
To use this command you need to know the root password. Only root can change the locale settings.
You can fire up "localeconfig" from within your graphical environment, but also in the terminal, just by typing localeconfig.