Whatever way you shake it, it would be disingenuous to claim to be fluent in a language when you really aren't fluent. It's possible you could get away with it, but probably not worth the risk. INSEAD says your native language is "...the language in which you consider yourself most fluent and skilled." You could find yourself in a very tight spot if your knowledge of Urdu came up during your interview.
If you already have some basic knowledge of French, I'd recommend simply resuming your study. It's possible to get up to B1 proficiency on the CEFR scale (required by INSEAD for entry languages) after just a few months of preparation. Also, although classes at INSEAD are in English, you'll almost certainly get more out of your time in France with a solid command of French.