I think xfce panel should have alternative default configuration which would include keyboard switcher applet present. It should be installed if the system language is (at least) Belorussian, Bulgarian, Greek, Russian, or Ukrainian, since those languages require the switcher, and it would save some user time needed to configure it manually.
The problem is this plugin is not provided with the panel itself, so it's not possible, well one could still add the plugin in the panel configuration but than there would be a zombie plugin in the configuration which is not clean. I suspect the right place for this request to be by Linux distributions.
I agree, we cannot add plugins that are not shipped with the panel.