See Wikipédia:Le Bistro.
Heddryin reported that the link “Brouillon” (which is “Sandbox” on enwiki) is sometimes displayed “Bac à sable” (which is the default value).
I had this bug two times today, the second time on an old version of a page where I noticed that the revision-info message had the default value too (which is why I think it is a problem of messages in general).