I'm not sure I agree with that. Not having a language means you don't have a framework for learning and organisation. Also, would your inner voice be available if you don't have a language? Most of my thoughts on this come from what I know about deafness, wrt language deprivation, and its effects on cognitive development. Certain things don't happen within a person if the brain doesn't get certain stimili at key points in development and the person becomes, in effect, stunted in the way they can interact with their environment or people. It's rather like, if a very young child receives no affection, or exposed to too many primary carers, their ability to form long-lasting social and personal bonds later in life can be seriously compromised and they may express life-long, socially pathological behaviour.