As anything in Magento adding a new language is something that requires a certain procedure which we will explain right now and for future reference.
Follow these easy steps and enjoy your multilingual site:
* 1. Download the desired translation thru Magento connect via admin.
* 2. Once installed Go to: System -> Manage Stores -> Create store view. Enter the data and save.
* 3. Now go to: Configuration -> Current Configuration Scope (Select your language from the dropdown) and on the right side under “Locale options” choose the desired language.