/dev/null

DOWNLOAD
Download the language packs for the language(s) you wish to make available. English is pre-installed; you do not have to download or install it.

What you download depends on your version of Joomla:

• For Joomla 1.5 each available language has two language packs, called "site" (frontend, what site visitors see) and "admin" (backend, what site administrators see). The language packs for many extensions depend upon having the relevant language installed in both the frontend and the backend, so it is usually best to install both language packs for each needed language.
• Beginning with Joomla 1.6, each language has a single pack with frontend and backend translations bundled together.

INSTALL
Install the language packs (1.5: Extensions - Install/Uninstall; 1.6 and later: Extensions - Extension Manager - Install). Click on "Choose" and select a language pack you downloaded. Then click on "Upload File & Install". Repeat for each pack to install.

Now, configure the language packs. At Extensions - Language Manager - Site, select the frontend ("site") default language. At Extensions - Language Manager - Administrator, select the backendend ("administrator") default language.

The language preferences of individual users can be set when they are registered, or at any time at Site - User Manager - username.

UPDATE
Whenever you update Joomla you should update your language packs. In Joomla 1.6 and later, compare your Joomla version with your language packs' versions at Extensions - Language Manager - Installed (Site) and Installed (Admin).

To update a language pack, simply install a new one as documented above.

REFERENCES
If you require additional localization beyond what language packs provide, consider Joomfish.