Function `_()` helps us translate strings. We first need to get the string translated and then replace format the arguments (i.e. replace %s with the arguments given). So this block should be like

```python
    msg = (_(u"%s's invitation to Mozillians has been revoked."
             u"You can invite %s again if you like.") %
           (deleted_invite.recipient, deleted_invite.recipient))

```

Please note where the parenthesis start and end.