Style comments, in no way binding:

Could pull the return tmpl.format() into the toplevel.  This works, by the way:

  user=msg['msg']['agent']
  title=""
  tmpl = self._("{user} unflagged an answer as offensive...")
  tmpl.format(user=user, title=title)

setting title could also be pulled out to the toplevel although it might be a little less clean if the 'askbot.post.flag_offensive.delete' doesn't set a msg['msg']['thread']['title'] at all:

try:
    title = msg['msg']['thread']['title']
except KeyError:
    title = ""

Once you've done that, you could also turn the if-then-elif logic into a hash but that might make readability worse (since there's 0, 1, or 2 more nested if-else's in any given if-then-elif levels) so I probably wouldn't go that far ;-)