Translate multiple deletion dialog

aleksis/core/frontend/messages/en.json

@@ -78,6 +78,7 @@
     "cancel": "Cancel",
     "close": "Close",
     "confirm_deletion": "Are you sure you want to delete this item?",
+    "confirm_deletion_multiple": "Are you sure you want to delete these items?",
     "delete": "Delete",
     "edit": "Edit",
     "save": "Save",