Joomla!'s translations are available in most major languages. However, you may like to change the translations and have your own translation in your desired language. In that case, Joomla! provides a mechanism to translate the Joomla! interface language. In this recipe, you will learn how to translate language files for your site from the administration backend.
Translation Manager is a popular extension that can help you translate the site's language files right from the administration backend, without opening a text editor. Download this extension from http://joomlacode.org/gf/project/joomla_1_5_tr1/frs/ and install it from the Extensions | Install/Uninstall screen.