Add initial support for xml:lang for streams and stanza plugins.

Remaining items are suitable default actions for language supporting
interfaces.

sleekxmpp/plugins/xep_0092/version.py

@@ -79,5 +79,7 @@ class XEP_0092(BasePlugin):
         result = iq.send()
 
         if result and result['type'] != 'error':
-            return result['software_version'].values
+            values = result['software_version'].values
+            del values['lang']
+            return values
         return False