Changing the default language can be done by editing the wp-config.php file of your WordPress website.
You can edit it by using simple text editor.
A corresponding MO file for the chosen language must be installed to wp-content/languages. For example, install xoousers-de_DE.mo to wp-content/languages and set WPLANG to ‘de_DE’ to enable German language support.
The “xoousers-” prefix should be specified.
Please note 2:
On WordPress 4.1 you may need to adjust the language settings in the WP Dashboard. Click on Settings – General and you will see the following screen.