The system message Wlshowtime ("Show last: ") appears in the watchlist header.
It is used to build a sentence by concatenation ("Show last: 3 days" or "Show last: 12 hours"), which makes it hard to translate. For instance, in French, "last" should be after the number and its translation depends on whether the unit is masculine or feminine ("Afficher les 3 derniers jours" or "Afficher les 12 dernières heures").
Possible solution: use something like "Period of time to display:" instead.