Don't try to translate IRC system messages

[tnibert]

MAIN temp: [':username@nat/ibm/x-dekwkdyfvvgijhzm JOIN #ubuntu\r']
THREAD An exception of type NotTranslated occurred. Arguments:
('Translation API returned and empty response.',)

We get the following exception and message when someone joins the irc channel. We are passing in the entire temp buffer to be translated. We should separate conversation text from the buffer to translate and not call translate when we have system messages like /join.