Sound, mother tongue and multilingualism
Language is a characteristic specific to humans. However speech is not granted or acquired this way or that. We used to think that our environment was responsible for our language, but since the past 50 years or so, research has widely shown that humans are programmed to communicate via language. It is a biological and genetic trait that obviously articulates with the child's environment to allow it to talk.
If we take some time to understand the above paragraph, we will easily be convinced about the importance of the mother tongue in child's development.
To add to this, in a technical and neurological approach, we need to understand that language starts with sounds. It is not surprising that till now, the first sign of life which we expect from a new born baby, is it's cry. Some do not hesitate to slap the baby at birth to hear those first sounds.
We also need to be conscious of the fact that the brain of a new born baby consist of an unlimited number sound receptors. The latter serve to store all that it hears and will be used later in his linguistic endeavors. However each individual language does not necessarily contain all these sounds. As a result the unused sound receptors will start closing 4 months after birth. This phenomenon is however not harmful to the development of the child's mother tongue.
On the other hand if we are aiming at developing the child's multilingual aptitudes, then we need to do our best to keep the sound receptors in the child's brain constantly open. We can accomplish this by exposing the baby to a variety of sounds, even including those which do not belong to its mother tongue. The use of onomatopoeias is a very interesting example but music from different instruments also works wonders.
This however demands that we try our best not to sabotage the child's environmental language by addressing him in uncertain and insecure terms. Let us be natural in our playfulness, conversations, laughs and songs to avoid ambivalence, fear and guilt while choosing between good or bad during its early years. You shall be witness to how it develops quickly in its mother tongue and other languages later on in life.