Document the module_lang option.

Closes #6169.

docsite/rst/intro_configuration.rst

@@ -310,6 +310,13 @@ different locations::
 
 Most users will not need to use this feature.  See :doc:`developing_plugins` for more details
 
+.. _module_lang:
+
+module_lang
+===========
+
+This is to set the default language to communicate between the module and the system. By default, the value is 'C'.
+
 .. _module_name:
 
 module_name

examples/ansible.cfg

@@ -22,6 +22,7 @@ sudo_user      = root
 #ask_pass      = True
 transport      = smart
 remote_port    = 22
+module_lang    = C
 
 # additional paths to search for roles in, colon separated
 #roles_path    = /etc/ansible/roles