Although the system seems to offer the posibility to change the language, the language pack is not included by default. So, go to this link to download the language pack.
1. download language files and place them into the right folders (app >design >frontend >default >default >local)
2. Login to admin panel and go to System > Manage stores
3. Create a new store view or modify
4. Login to admin panel and go to System > Configuration > General
and select each store view from “Current Configuration Scope” drop down menu at the upper left corner of the window.
Set the corresponding locale for each store view
5. Clear the cache (System – Cache Management)