Languages are not equally well supported in all packages using GNU
gettext, and more translations are added over time. Usually, you
use the translations that are shipped with the operating system
or with particular packages that you install afterwards. But you can also
install newer localizations directly. For doing this, you will need an
understanding where each localization file is stored on the file system.
For programs that participate in the Translation Project, you can start looking for translations here: http://translationproject.org/team/index.html. A snapshot of this information is also found in the ABOUT-NLS file that is shipped with GNU gettext.
For programs that are part of the KDE project, the starting point is: http://i18n.kde.org/.
For programs that are part of the GNOME project, the starting point is: http://www.gnome.org/i18n/.
For other programs, you may check whether the program's source code package contains some ll.po files; often they are kept together in a directory called po/. Each ll.po file contains the message translations for the language whose abbreviation of ll.