Add note about LocaleListener

LocaleListener uses information provided by Request object. Setting it after LocaleListener read the object results in no effect.

Author: Steveb-p

translation/locale.rst

@@ -25,6 +25,12 @@ it::
             // some logic to determine the $locale
             $request->setLocale($locale);
         }
+.. note::
+
+    Using a listener like this requires that it is called **before** 
+    LocaleListener attempts to access it. In Symfony Framework it is called
+    with priority 8 by default. You will need to set your event listener
+    with a higher one if you want this to work.
 
 Read :doc:`/session/locale_sticky_session` for more information on making
 the user's locale "sticky" to their session.