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 back end installer. After this, go to the extensions menu, select language manager and make your language the default one. Your back end will be translated instantaneously.
Users who have access rights to the back end may choose the language they prefer in their personal details parameters.
This is of course also true for the front-end language.
A good place to find where to download your languages and localised versions of Joomla! is on our Help Site.