Brion Vibber wrote:
Unless someone's requested an explicit overwrite run, only messages either not present or last edited by 'MediaWiki default' (ie those created by the installer or periodic upgrades and refreshes) are automatically updated. If something's been auto-updated that shouldn't have, perhaps either it's a bug (in which case details always help to track it down!) or the language file has changed that way and it's a legitimate update from the software's point of view.
Avar recently ran it with --rebuild on ruwiki in order to fix a few blank messages. This drew complaints, and from what I know of the facts, rightly so. Maybe he did it on cy as well.
-- Tim Starling