How to install a different language

To install a locale, simply enable the locale module. Then, go to admin/localization and import a .po file that you downloaded from http://drupal.org/project/Translations. You should select the right language code in the popup menu. That's it !

You can now let user switch languages, or not, by activating more than one. Also, you could look in the tab "translate srtings" in the localization module.

See Locale or internationalization support for more.