I just discovered there is a way as 0.15.0 to add a flag representing the keyboard layout. I believe the old LXDE Lubuntu used to do this. Here's how:

1. Add the kbindicator widget to the panel
2. Install the iso-flags-svg package
3. Change the "file path pattern" to "/usr/share/iso-flags-svg/country-4x3/%1.svg" (or replace "4x3" with "squared")

I think we should make that all default, albeit with one change: the widget defaults to having indicators for caps lock, scroll lock and num lock. I don't think they're terrible, but they do take up some screen space, so I think we should flip them off.