Translate WordPress in Your Language
WordPress has one of the largest if not the largest communities in the world of CMS applications.This community has translated WordPress into many languages and you can easily change the default language of your installation.
To change the WordPress back and frontend language, open the wp-config.php file in your root folder.
In it locate the following line:
define ('WPLANG', '');
and replace it with:
define ('WPLANG', 'bg_BG');
where bg_BG is the locale code of your country.