It would be surprising if a child growing up in an English speaking home suddenly started speaking Urdu, with no one at home able to speak a word in Urdu. A child’s primary language therefore develops from exposure rather than formal learning.
Other languages may be learntformally or informally, but the primary language is that which forms the foundation from which other languages may later be gradually understood. Likewise individual reflex behaviours are developed from primary learning. No wonder it is said that a fruit does not fall far from its tree.
What language does a child speak? Anger, joy, love, hatred, gladness or sadness, aggression or kindness, warmth or cold? If the child is not speaking the home language where may the child have been exposed to the “foreign” language spoken?