If you have set the keyboard layout system-wide, you do not need to do anything. If, however, you switch between layouts on a regular basis, Fluxbox lets you easily add this functionality to the menu.
1. Open the Fluxbox menu config file:
nano ~/.fluxbox/menu
2. Add the following section with the layout(s) you want:
[submenu] (Keyboards)
[exec] (gb) {setxkbmap gb}
[exec] (fr) {setxkbmap fr}
[end]
3. You'll also need to reload the config.