WordPress needs to be told which language it should run under.
WordPress v4.0 and Above
Change the language in the admin settings screen. Settings > general > Site Language.
WordPress v3.9 and Below
Open wp-config.php and modify the line below, or add it if it is missing. We’ve set it to Norwegian in the example below. See the complete list of language codes and country codes to find your exact locale.
define ('WPLANG', 'nb_NO');
Once this has been done, if the language files exists the site will be displayed in your locale. Otherwise you will need to create some language files and translate the text strings (see below).