[runarberg]

Excellent observation. In fact the language part of the brain is only a (albeit a rather large) portion of the brain (not nearly as large as visual processing though). And people who suffer brain damage which renders them unable to speak (or understand speech; which interestingly is a different portion; albeit close to each other) are still able to demonstrate intelligent behavior with ease.

In fact it is damage to the prefrontal cortex (which has nothing to do with speech) which is mostly correlated with a detriment in intelligent behavior (suspiciously also social behavior; a food for though in what we consider “intelligence”). Victims of lobotomy had their prefrontal cortex destroyed, and their injuries resulted in them loosing their personalities and loosing basic function as human beings, even though they were still able (but perhaps not always willing) to speak and comprehend speech.