Mac (Carbon): find out keyboard layout?

[David Reitter]

Is there a way (on the Lisp level in the 22.x Carbon port) to  
determine the currently chosen keyboard layout?

I tried to

(define-key special-event-map [language-change] 'my-function)

to keep track of the keyboard layout (e.g. "Italian Pro", "US  
English", etc.) but my-function doesn't seem to get called.

Any hint appreciated.