add findLocale() to public interface

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/public/L10N/IFactory.php

@@ -44,6 +44,13 @@ interface IFactory {
 	 */
 	public function findLanguage($app = null);
 
+	/**
+	 * @param string|null $lang user language as default locale
+	 * @return string locale If nothing works it returns 'en_US'
+	 * @since 13.0.0
+	 */
+	public function findLocale($lang = null);
+
 	/**
 	 * Find all available languages for an app
 	 *