Our daughter is half Tamil and half Gujarati. She's exposed to both languages in almost equal measure, through my wife and my parents. She's five now and speaks predominantly in English, understands Tamil, shows frustration in Gujarati and speaks Hindi with a distinct South Indian twang. I really don't know what her mother tongue would be. I guess it is best for her to decide what language she identifies with the most. I am neither a linguist, nor a purist. I believe that language is just a medium of expressing one's thoughts and beliefs. As long as she has the ability (and courage) to speak her mind coherently and convincingly, I really don't mind which language she chooses to do it in.
That said, we end up learning a lot in life, but it's always good to remember where it all started from. Because if we don't know our roots, how will we know where we stand? So, on February 21 - World Mother Tongue Day, be sure to express yourself... in the language you are most comfortable in.