Has you defined the language constant in the wp-config.php file,
* 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.
Following post might be helpful to you,