If you are in Switzerland on a UN legitimation card and permit, then you do not pay the same taxes the rest of of pay. I understand that Geneva doesn't hold this as a obstacle to naturalization but Vaud does. But in neither case does the fact that you have put in your 12 years means you get it free and clear.
It is 5 years for the French passport, or four years if you are married to a French but living in Switzerland, and less (three?) if you are married to a French living in France. However, one of the conditions for applying for French nationality without it being through marriage is that your center of interests be in France (professional, personal, etc). UN employees may not automatically be assumed to have interests based in France.
What I am trying to say is that I am not entirely sure that just putting in your time while being on a legitimation card is enough in either France or Switzerland, because of the tax issues in CH and because the French could argue that your center of professional interests is not in France. It could happen but it could not.
All I can say is good luck, I'm fighting for my French passport myself as my Swiss one won't come down (if I am lucky) until 2017, because I didn't marry well like Michele Bachmann