Traditionally the mother spent more time with a child; it is not surprising that the term ‘mother tongue' is used.
Research has shown that much of mothers' language interaction with their children is about basic housekeeping functions (e.g. feeding, bathing, dressing, discipline). Fathers have opportunities to play with their children, allowing considerable language stimulation. Many fathers interact with their children in child-centred ways. Thus a father's contribution to a child's language development is sometimes underemphasised. Thus fathers need to be aware of the important role they play in child language development. Leisure time has grown over the years. In such situations, the role of the father in language development has become more important.
In some families (e.g. where one parent speaks one language, the other parent a different language), both parents promote bilingualism. In this situation, fathers and mothers need to be conscious of their important role in the child's language development. Both parents should be encouraged to take pride in their conversations with the child, even at the babbling and cooing stage. Even before the child says its first word, the baby picks up the sounds of the second language from the both parents. Both parents need to vary the contexts in which language is used to give that child a wider language experience.
Both parents influence the attitudes of their children to languages. For example, whether the father is positive or negative about bilingualism will considerably affect the child. If the father is sceptical about bilingualism, or doesn't like the mother using her ‘own' language, the child will soon pick up these negative vibrations and language behaviour will be affected. On the other hand, if a father encourages his children's bilingualism, applauds them speaking to their mother in her ‘own' language, the effect on the child's language confidence and attitudes will be substantial.
An important decision between parents concerns what language to use with each other when the child is present. Sometimes that language is naturally the one always used in the partnership. Yet for the child's sake, consideration needs giving to achieving an approximate balance within the family between the two languages.
Continuity in minority language experience from early childhood through the teenage years is important if that minority language is to survive through to early adulthood.
Therefore, the answer to the question is that both parents are very important in the child's language development. Both parents need to be aware of the importance of the language that the child hears and uses with each parent.