If the setxkbmap is not installed on the system, then it is possible to install xfce4-xkb-plugin but it does not work. It means that the plugin icon displays on the panel but if you try to switch the keyboard layout then the keyboard layout remains unchanged. If you later install setxkbmap, then the xfce4-xkb-plugins works as expected.
+ 05 Jul 2015; Jason Zaman <perfinion@gentoo.org> + +xfce4-xkb-plugin-0.7.1-r1.ebuild: + bug 528172, add RDEP on setxkbmap