Update:

When I picked up the car, they did mention that very few of their cars at CGD have the GPS system. It was a Peugeot. Diesel that got good milage, but had difficulty making it up steep hills with any speed.

After many days of driving around listening to the French commands, for some reason I was digging through the glove box and guess what I found buried under the car manual and various Hertz literature? A new language CD with multiple languages, wrapped neatly in plastic and unopened. After quickly loading it up, the trip from then on was much improved in terms of navigating around. From then on, the system spoke with a much more understandable female British accent.

Of course, she still couldn't help me find which terminal I was leaving from -- and thus, which terminal to drop the car off at. They really could use a sign to indicate which airlines fly out of each terminal.

With about 20,000 km on the car I guess I was the first one to drive it who needed anything but French. Unfortunatley for whoever rents it next, I did leave the system running in English -- hopefully they don't have as much of a problem changing the language back as I originally did!