The books that I have read (The language instinct, The Symbolic Species et all), put forward the theory (which makes sense to me) that the ability to acquire language, or rather the raging hunger to inhale large quantities of verbiage, is innate in the human child's developmental process.

However, the language that is learned, be it English, Chinese, Tagalog or sign language, depends upon the environment in which the child grows up.

Language is not just another skill that is just learned in the same way that a general purpose computer can run any program. Our bodies, particularly throats and brains, are highly adapted and specialised to the tasks of language.

Steven Pinker points out that the adaptation goes the other way too: human language is by definition the words and rules that have survived being learned by countless generations of children, and therefore whatever is left must be suited to being learned by human children. Thus our languages do not vary without limit - Each language feature fits into one of relatively small number of categories because we have the mental equipment to be able to learn certain kinds of languages.

You may well ask: Do birds learn to fly, or is it innate?

taschenrechner answers the rhetorical question: Yes we have to learn to use the brains and throats that are designed for language, just as birds have to learn to use their wings thought they are designed only for flight. In both cases, the skill is both learned (baby birds can't fly, having neither the skills nor the strength) and innate (would not get far without those wings).

With regards Cognis' excellent write-up, I think it is fair to say that something that is "entirely learned" but learned over timescales over which evolution plays a significant role, so that special structures for the ability have become part of the body, has in some sense become part innate. Cognis is also correct that the simplest explanation is that we started speaking with simple calls, like monkeys or birds, and that complexity of speech co-evolved with better speaking apparatus to enable that speech.