People naturally learn language. A child will learn language if around people who speak. A monkey will not learn gestures if around humans who gesture, unless those humans intentionally train the poor thing to mimic them.
Do you know this to be true? Have you any experience of anyone who has raised a human child without speaking directly to the child, prompting the child to respond?
Do I know what to be true?
I have no experience, nor even second-hand knowledge, of anyone who has raised a human child, which child used a human language, without speaking to the child. It may have happened; I just don't know. I will concede that a child must be spoken to
in order to learn language. But, again, I don't really know if that is true. I am conceding it here because it doesn't matter. The child begins to learn and continues to learn language independently of formal training in it. In addition, I have read reports of children raised by deaf parents who learned Sign. I suppose that besides the Signing they did "directly to" the child, the child learned some Signing on its own independently of being directly taught. (Teaching is different from training, you realize?)
I do not believe people learn language (spoken or Sign) unless they are around others who use language. (I will concede what seems to be an important point to you, that the language must be directed at the child.) There have been cases where isolated children had been found, who apparently had created/developed no real language on their own. They grunted and screeched, and that was about it.
Monkeys, however, have been around people who speak "directly to" them, but they don't talk back. Never. They don't even Sign back to people who are deaf who use Sign to communicate with each other. Never.
Some new development may overthrow this, but I will not be persuaded by anyone's unsubstantiated religious passion
that it "should" be so.