Replace US keyboard with french keyboard

This is a big job replacing the keyboard on these systems. Follow this guide to see how big: MacBook Air Models A1237 and A1304 Upper Case Replacement. Depending on your unit you will have aprox 85 screws and/or snap rivets to remove the keyboard if you replace the keyboard alone or just the full top case assembly (which is what I do). In addition the battery is glued to the case so you will need to replace it as well.

As to which language you get either will work. You'll need to changeout the keyboard layout driver via the control panel for the correct one for your keyboard.

If you have a working keyboard I would strongly recommend just swapping out the keycaps as a cheaper solution here. Or, just get an external Bluetooth keyboard if you need to switch over often.

As you stated your keyboard is broken I would first see if it can be fixed without replacing. Often a little cleanup is all thats needed if you spilled something into it. I would start there and consider just using an external wired or wireless keyboard if you can't fix the internal keyboard.