A child learns its mother tongue from the womb itself. During the pregnancy, all the conversations of its mother is heard by the child. When it comes out from its mother's womb, it can easily recognize the words and phrases from her mother. The child develops fluency in its mother tongue. Supposing, if the mother is highly educated and speaks English at home, her child would be a British with good English. So, mothers should speak in their own mother tongue or the local language, if they want their child to be expert in the mother tongue.
For a child, it is necessary to learn its mother tongue, to cope up and mingle with the children around. If not, other children will consider the child as British and would keep the child in isolation.
No life without Sun