Speech is not central to language. What is language?
A language is a system of relatively arbitrary symbols and grammatical signals that change across time and that members of a community share and use for several purposes: to interact with each other, to communicate their ideas, emotions, and intentions, and to transmit their culture from generation to generation. -- (Cokely et al, p 1)
Most of us are familiar with the fact that the left region of brain is activated when a language is used.
What about the visual-spatial speakers? Eyean speakers? English or any spoken language is a vocal-aural language, whereas ASL or any signed language is a visual-spatial language. These modalities are quite polar, yet languages (signed and spoken) are no different at all in term of natural human language and in biology.
"The human brain does not discriminate between the hands and the tongue. People discriminate, but not our biological human brain." -- Dr. Laura-Ann Petitto (Gallaudet Today, Spring 2012, p. 17)
Basically, visual-spatial languages and vocal-aural languages activate in the same regions of the left brain (Wernicke and Broca). Second, both are on the same timeline in language acquisition (L1).
What does studies tell us about cochlear implants and speech? This Gallaudet's video below explains all important information in short on the brain, language, modality (signed and spoken), language acquisition, and myths and facts on cochlear implants.