To install the other language to the extension you should:
- Download the propriate extension's language pack from our downloads section
- Install the package as a normal Joomla extension using Extension Manager
- Set your default language in: Extensions->Language Manager (for frontend or backend or both)
Worth to know:
Starting from Joomla 2.5.7 version you can install Joomla language packages using the built-in mechanism, all you need to do is in your Joomla backend navigate to Extensions->Extension Manager->Install Languages then type the language name and install the language package from there.
If you want to know more about setting up the multilanguage site in Joomla check this article: http://www.joomla-monster.com/blog/joomla-2-5/need-help-to-configure-you-multilanguage-site