How innate is language?
Language can be acquired spontaneously through observation.
Language is somewhat not inborn nor does it comes naturally. It is learned through observation. Even if a pure German baby is born in America, that baby will learn to speak English if his/her surrounding people are English speaker, not German.
Almost all human beings acquire a language to the level of native competency (where full conversation can be induced) before age 5.
According to most researchers, children are born with an innate biological "device" or whatever they called it, that helps human beings understand the principles and organization common to all languages. In this theory, the brain's "language module" gets programmed to follow the specific grammar of the language the child is exposed to.
There is greater emphasis on the influence of usage and experience in language acquisition. This means that adults play an important role in language acquisition by speaking to children, often in a slow, grammatical and repetitious way. In turn, children discern patterns in the language and experiment with speech gradually, uttering single words at first, and eventually stringing them together to construct abstract expressions.
They may not learn it quickly and easily as children seem to. Even on adults, if an adult may stop using a language, there is a possibility that he/she may unlearn that language. So language needs constant usage.
Hope that helps. :-)