Language variations (en-GB, en-AU, etc.)

Suppose some content has variants in en-GB, en-AU, etc, as well as plain en. If a user's language preference is en-GB, they should see en-GB content if available but if not they should see the standard en version. I am not aware of any way to do this with existing modules (though it wouldn't be very difficult to write.) Am I wrong? Is there some way to do this already? (with D6).