I strongly believe that mother tongue is the most important. Whilst many people can speak and write English, that does not mean that their mother tongue is English. So I only talk to my daughter in Cantonese when she's small. I do, however, let her listen to English songs and watch English DVDs. I think I only start "talking" to her in English after she started school. It's very obvious that my daughter started to speak complete sentences (Cantonese) at a very early stage and she's strong in understanding and analyzing things. She's also quite good at mathematics.