If you want to switch keyboard layout on the fly, this must be done on the client side. In some cases, the client OS provides GUIs and/or hotkeys for this. However, when running in fullscreen mode, you cannot typically use them: The GUI is not visible, and the keyboard might be grabbed. One idea is to provide our own hotkey or GUI element for this. For example, we could provide an option on the "F8" menu.
On UNIX, we can probably expect that "setxkbmap" can be used. On Windows, there are API functions such as ActivateKeyboardLayout. OS X remains to be investigated.