I have five kids and it still never ceases to amaze me with each one how they develop their native language. It's not too difficult to understand how one can learn a second language or third or fourth language, but learning a language with no language is mystifying.
Typically, we think that children learn the meaning of terms by ostension--by some sort of pointing children learn what is signified by the sign or word. Some of the first books we read to our kids are picture books with shapes, objects, and colors. You point to a red truck and say "red truck," point to a red ball and say "red ball." Eventually the kids learn to see what is in common when the word "red" is used and the like.
But then (to borrow from Quine) how do children learn my meaning when the range of possible meanings seems underdetermined by the objects pointed to? I point to a rabbit and say "rabbit." "Rabbit" could mean that furry animal which is alive (what you and I MEAN by rabbit!), it could signify undetached-rabbit-parts, the space that is always inhabited by a rabbit, the outside of a rabbit, and so forth. Or take "walking." I could walk around the room and say "I am walking." But how does one recognize walking from hurrying, taking 50 steps, sauntering, moving, and the like? How does a child recognize that anyone who has walked farther than I have when I have said "I am walking" is also walking? How is it possible that language gets started? Yet children seem to automatically (or at least very quickly) understand what I mean be the term.
I am reminded of Augustine's Confessions where he describes his own learning of a language as a child (1.6.8)
One thing that is interesting is how Augustine describes himself as having a capacity to think and reason (to some extent--at least reasoning about certain means-to-ends) long before he has a language.
Little by little I began to be aware where I was and wanted to manifest my wishes to those who could fulfill them as I could not. For my desires were internal; adults were external to me and had no means of entering into my soul. So I threw my limbs about and uttered sounds, signs resembling my wishes, the small number of signs of which I was capable but such signs as lay in my power to use: for there was no real resemblance. When I did not get my way, either because I was not understood or lest it be harmful to me, I used to be indignant with my seniors for their disobedience, and with free people who were no slaves to my interests; and I would revenge myself upon them by weeping. [Now that is funny!] That this is the way of infants I have learnt from those I have been able to watch. That is what I was like myself and . . . they have taught me more than my nurses with all their knowledge of how I behaved.
Ultimately, at bottom, I think Augustine believes that one can only learn a language because of God-given intelligence or divine light which one has prior to any language acquisition. There are of course alternative explanations, and not all will agree that there can be thought prior to language. It seems to me, however, that it might be a greater miracle if language can develop with no prior thoughts.