A lot of different languages are available for the Back-end, but by default this language may not be installed. If you want a translated Back-end, get your language pack and install it using the Extension Installer. After this, go to the Extensions Menu, select Language Manager and make your language the default one. Your Back-end will be translated immediately.

Users who have access rights to the Back-end may choose the language they prefer in their Personal Details parameters. This is of also true for the Front-end language.

A good place to find where to download your languages and localised versions of Joomla! is Translations for Joomla! on JED.