1. download language files and place them into the right folders (app > local)
2. Login to admin panel and go to System > Manage stores
3. Create a new store view
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)

In order to set the default language for magento, you must go to System > Manage Stores, click on the “Store Name” and set the default language from there

Beware of the content of your language pack. It must have been tested that it works properly.

I went to php.ini and enabled that extension without luck.

It turned out that you must have also “php_pdo.dll” enabled in order for the installer to proceed.