The general issue there is whether there is any language's dependence on general intelligence. A double dissociation would suggest independence. Different populations in which there are great linguistic ability in the presence of severe cognitive deficits (hyperlexics, chatterbox kids). Other populations where a brain damage occasion a language deficit independently of the rest of the cognitive profile (deaf people, aphasics, specific language impairment).
In order to show that humans have a genetically-based linguistic endowment that is not simply a by-product of general intelligence (or is it just that Having language localized in a specific area of the brain predicts that?), different populations have to be found in with:
impaired linguistic ability with regular intelligence
Most commonly, the issue of whether language and cognitive level are out of step has been investigated through SLI (Specific Language Impairment), where otherwise normal children fail to acquire language, most famously now in the KE family brought to media attention by Myra Gopnik. But even here it is unclear whether cognitive faculties are truly unimpaired in people with SLI. According to Vargha-Khadem, Watkins, Alcock, Fletcher, and Passing-ham (1995), among other differences the average IQ of the SLI members of the KE family was 86 compared to 104 for the rest.
regular linguistic ability with impaired general intelligence or in the presence of severe cognitive deficits
A strong case for an innate language faculty distinct from general intelligence could be made if there were individuals who have acquired the highly complex system which we call grammar, without parallel cognitive abilities of equal complexity. There are now a number of such studies of children who have few cognitive skills and virtually no ability to utilize language in sustained meaningful communication and yet have extensive mastery of linguistic structure.