Has you defined the language constant in the wp-config.php file,
[PHP]/**
* WordPress Localized Language, defaults to English.
*
* Change this to localize WordPress. A corresponding MO file for the chosen
* language must be installed to wp-content/languages. For example, install
* de_DE.mo to wp-content/languages and set WPLANG to ‘de_DE’ to enable German
* language support.
*/
define(‘WPLANG’, ”);[/PHP]
Following post might be helpful to you,
http://www.wikihow.com/Change-the-Default-Language-in-Wordpress
Thanks