WordPress can be used in many different languages. By default the language installed and active on WP sites is English. You can easily change your installation language by editing your wp-config.php file which is present in your site’s root directory. Remember it may happen that the language you have defined in your wp-config.php file is not installed in your WordPress site. In such case you can download your language files from here and upload them to Your site root -> wp-content -> Languages directory.
For changing language of your WordPress installation, visit your site’s cPanel or FTP then open your wp-config.php file and you can define your language here as shown below:
For using English:
define ('WPLANG', 'en_EN');
For using Deutsch:
define ('WPLANG', 'de_DE');
For using Italian:
define ('WPLANG', 'it_IT');
In the same way you can add your language, take help from WordPress code here for learning more about WordPress languages hack.