Of course the prices are slightly higher at the airport and much higher at Cornavin. In addition, you have to pay for the paper or plastic bag, so keep them and use them again. I guess this is their way of recycling, or at least reducing the amount of paper and plastic used. Try not to buy too much food and then have to throw a lot away when you leave. Remember, you cannot bring food into the United States.
Swiss chocolates are okay.
Finally, if you have a favorite snack food, I would suggest you bring that. They do not have the same types of food and again, it is expensive. In Geneva, French is the predominate language, although Geneva is a very cosmopolitan city given all of the international organizations based there. You should be aware that there are some people that do not speak English at all. It is always best to start out with a greeting and then ask if they speak English.