Nail polish is a product that is used to literally paint a specific color on fingernails. Acetone, also known as nail polish remover, is the proven and accepted way to remove nail polish on nails and most other hardened surfaces. It will also work when removing polish on leather goods as well, however the leather must first be determined to be colorfast before using acetone.
Dab off, don't rub, most of the nail polish using a paper towel. Then test the leather for colorfastness. This will help you determine if the leather can withstand the chemicals needed to clean it. Take some acetone and dab an area of the leather that cannot be seen or is inconspicuous. Acetone is a non-oily fingernail polish remover. Wait a few minutes to make sure the leather does not change color.
If the leather can withstand the acetone, dab the affected area with small amounts of the acetone using a paper towel. Keep dabbing until no more nail polish is coming up on your paper towel. If the acetone damages the leather you can try using rubbing alcohol. Be sure to test the alcohol first before using. Use the same method of dabbing.
If neither one of these methods work on your leather, contact a leather professional. They have access to more chemicals and should be able to remove the stain.