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.
Note: Please clear the cache before trying to use the new language.
Now, on the frontpage, you'll notice a dropdown menu allowing the language switching.