Now let’s look at language learning. Learning a language for a young child is, it seems to me, the most difficult and demanding learning project any human being ever accomplishes. Even learning to read is a no-brainer compared to learning to understand and speak a language.The first obstacle is to get what language actually is. The story of Helen Keller in The Miracle Worker was about her coming to finally understand what language is, that the shapes made by the hand and fingers of her teacher, Annie Sullivan, represented objects, actions, relationships, people - the world itself! No language, no (meaningful) world. It may take babies a year or more to get what talking is other than not-yet-understandable but non-random streams of vocal sounds. This stage is critical for the beginning of an understanding of meaning, and why talking to and around your baby, in a normal conversational way, is so important for early language learning. This early language experience is hard to make up once school begins and can begin at birth as babies learn facial expressions, gestures, etc., associated with specific vocal sounds they hear. Read my post on Language Dancing (Dr. Todd Risley).
Consider the difference for us hearing people to learn to understand a language by listening to a stream of sounds the way it is for those of us without hearing to learn sign language or to learn to read for the following reason: in both sign language and written language, the meaningful signs and written words are already distinct from each other - all the time. In audible language, individual words are seamlessly attached to each other in an ongoing stream which immediately disappears. And a human brain has to, frequently if not mostly on its own, learn where to break the streams and attach meanings in a continuous stream of ongoing and immediately disappearing data. This might be like learning to read if you saw a stream of words that disappeared as soon as you saw them, with no spaces and no sounds, as you watched someone interact with you and do other things. (On the other hand, it is possible that our essentially infinitely large computer-like brain can begin distinguishing strings of 0s and 1s - binary streams of electrical charges - collected simultaneously and linked in time from our five senses, and then, through trial and error, it organizes them into meaningful databases.)
So how do we measure language ability?
Give an oral test and/or a written test? How many children may talk a mile-a-minute at home and not in public? Older children may talk a mile-a-minute with peers and not much at home, yes? How many adults have a fear of public speaking? How does being assessed by an authority that can "tell" on you, that can punish you, that can take away your play (work) time if you don't "measure up", that can fire you, affect what your level of communication? See the problem?
Early language learning is almost all a result of the quantity and quality (friendly, informative, and fun) of social conversations at home, and helped by including adults who enjoy talking respectfully to young children. See Language Dancing for Babies & Parents blog post.