There are five GTK+ stock strings in xfwm4 that have been marked as translatable. In a thread on the xfce-i18n mailing list I was told that this should not be the case and was asked to file a bug. Individual stock strings should only be marked translatable if a translator requests this due to specific needs of their language.
The strings are as follows (in PO template format):
Thanks, fixed in revision 29030.