A babies first six months doesn't seem much, but scientists have discovered that babies can distinguish between different languages based on the facial expressions as one speaks. The article from New Scientists has two videos with no sound, one in French and the other in English, so readers can see the changes in the face when the two languages are spoken. Interesting enough, bi-lingual babies can still pick out the native and foreign tongue, whereas mono-lingual babies stop at six months. I hope this does not start eager parents to start speaking in many different languages so their babies can be mutli-lingual.
For me, it's only natural to speak to your own baby. For me, I will speak to the baby in Cantonese and English.