Add warning about using the correct language codes

Author: yakky

docs/getting_started/configuration.rst

@@ -275,6 +275,12 @@ Example::
 
 .. note:: Make sure you only define languages which are also in :setting:`django:LANGUAGES`.
 
+.. warning::
+
+    Make sure you use **language codes** (`en-us`) and not **locale names**
+    (`en_US`)here and in :setting:`django:LANGUAGES`.
+    Use :ref:`check command <cms-check-command>` to check for correct syntax.
+
 ``CMS_LANGUAGES`` has different options where you can define how different
 languages behave, with granular control.