[Reference] Add default_locale config description

reference/configuration/framework.rst

@@ -19,6 +19,7 @@ Configuration
 * `http_method_override`_
 * `ide`_
 * `test`_
+* `default_locale`_
 * `trusted_proxies`_
 * `form`_
     * enabled
@@ -151,6 +152,16 @@ This setting should be present in your ``test`` environment (usually via
 
 .. _reference-framework-trusted-proxies:
 
+default_locale
+~~~~~~~~~~~~~~
+
+**type**: ``string`` **default**: ``en``
+
+This locale is used, if no ``_locale`` routing parameter has been set. It becomes
+the service container parameter named ``kernel.default_locale``. It is also available
+with the :method:`Request::getDefaultLocale <Symfony\\Component\\HttpFoundation\\Request::getDefaultLocale>` 
+method.
+
 trusted_proxies
 ~~~~~~~~~~~~~~~