Wordpress add filter locale

wp-add-locale.php

if ( ! function_exists( 'wolf_set_lang' ) ) {
	/**
	 * Set your language here
	 *
	 * The .pot file can be found in the languages/ folder of the parent theme
	 * http://help.wolfthemes.com/2013/11/translate-your-wordpress-theme/
	 *
	 * @param string $locale
	 * @return string
	 */
	function wolf_set_lang( $locale ) {
		return 'fr_FR'; // this is your Locale Code see http://wpcentral.io/internationalization/
	}
	add_filter( 'locale', 'wolf_set_lang' );
}