(When reading the following, please keep in mind that it is a single persons view; it is not meant to end the debate of nature vs. nurture, or any other debate. It simply states the belief of a person, me).

Language is entirely learned. Our bodies have, most likely in response to our ancestors use of language, developed very intricate organs for use in language: Our vocal cords and ears. These allow us to perceive and use spoken language, which in turn has been supplemented by symbolic language, in most cases writing. Our dependency on communication has forced those without natural ability (the mute and/or deaf) to construct complex substitutes like sign language (for the blind, braille allows reading and writing. More exotics systems exist). However, theoretically, a human being can live all its life without ever developing any language, because it is a skill learned.

The reason I feel so sure about stating this (well, one reason) is the discovery of wild children. I particularly remember a case of a girl being discovered at the age of 6 or 8 living with (no fooling) pigs. Apparently, she had gotten lost or been abandonned at a very young age, but had survived by imitating the behaviour of pigs, allowing her to find food as they did. I no longer remember many details of the case, but one important factor was, that she had no language what so ever, knowing only the basic gestures and sounds made by swine. All her organs for developing language were, however, entirely intact, and with help she developed basic language skills over time.

Children learn primarily by imitation. I have personally (as has probably any parent or person working with children) witnessed small children engaging in conversation with each other, without having learned to talk yet. Quite simply, they see and hear people (especially parents or similar adults on which they depend or with whom they spend much time) talk, and they copy the behaviour. Since a child at that point will have no clear-cut expectation of results from 'saying' something to another child, they can seem to have a full conversation in 'baby tongue', when in fact they are simply improvising, acting out what they have seen others do. The 'pig-girl' never experienced adults talking, and thus never learned until being taught by her rescuers. She 'had the wings', but she had never 'learned to fly', to use the bird-comparison.

In our modern world, communication is very important. It is also very common. This alone is sufficient to make those without the proper organ functions develop simple substitutes for spoken language. In addition to that, the human body is built in a way that gives the creative mind many ways to communicate. However, without outside influence, a human is unlikely to develop any sort of language skill above what other animals do (growling at an enemy, screaming in pain or discontent). Only if someone is actively trying to understand the child (requiring someone with a developed concept of language to be present) will it have a chance to experiment with a basic 'language'. We learn by imitation; no language to imitate = we dont acquire/learn language.

As a note for those thinking "hey, language had to develop at some point", the answer is: Yes, it did, we did not learn it from aliens (I think). However, it didn't just pop up, but rather started out as growls and grunts, which got more varied over hundreds of thousands of years. Our ancestors gradually turned this into basic language, and at some point, they realized more advanced uses for it. This does not imply that language is innate to humans; we eventually developed bungee jumping and teletubbies as well, but no anthropologist will claim those to be inevitable developments, either.