I think anything that will take out the nail polish (nail polish remover, rubbing alcohol, etc.) will also probably take the color out of the leather. So, its probably a question of what is worse to you - nail polish on your bag or a light spot on the leather.
I had a similar situation a while ago when I got ink on my Balenciaga. I used a little alcohol on a q-tip and it took the ink out, but it also made the leather lighter (which was ok with me, since it was a lighter color leather anyway).
I so agree with this! First, step away from the bag and fight off any urge to do it yourself. You will only make it worse. I committed great bodily harm to one of my Balenciaga bags the same way. If you don't want to send it to Fendi, there is Barbara at Lovin My Bags or Artbags in New York who can professionally clean it for you. Some people have had great luck with cobblers too. I would recommend Fendi since they are the manufacturers. It may take some time, but you know that they know what they are doing and if they completely screw it up, they have the capacity to give you a replacement or fix it. Good luck and keep us posted! In the meantime, hang in there and fight the urge!