I can imagine it working like this:
the CR could support notifications as Malte suggests, using the :best available method".
If a content type desginer doesn't provide the correct service contract definition, the fallback would be a plain text email in the language of the user, saying something like "The item named Foo of type Bar was modified at this time/date by this user. Click here to view the changed item."
A service contract could be written for any types desiring special handling (for example an image might include a thumbnail in the email, or the type "bug" would be handled in the way it is now).
The output of the SC would be a chunk of text or html in a form suitable for joining together with each other, for the case that a user has requiested digested notification.
