Fallback to cs
Thanks, but it seems not to match reality in a few random tests I used. E.g. Notification-add-comment-yours2 is listed in sk and not cs, while it exists in both (and is an outdated/unused message anyway). Dtto Centralauth-editgroup-subtitle or tpt-desc. OK, User-profile-preferences-emails is in sk but not in cs, but it is unused anyway. Dtto ga-threshold. Whoa! Finally I have found Security-restricttogroup which seems to be a correct example. But the overall validity seems to be not very good by this random test.
Anyway; as I have said, I am not against the idea, just that I do not consider it that much important or useful. If someone does, by all means, go ahead.