Somewhere in The Language Instinct, Stephen Pinker discussed research on late first language acquisition. These are deaf people who are not taught sign language as children.
At the time I read this stuff, I did a literature search and didn't find any philosophers talking about these people, and the scientific literature was also pretty narrow. There was a great deal of developmental research showing that the "critical period" hypothesis concerning second language acquisition (that around the age of eleven it gets much harder to learn a second language, and progressively harder as the learner ages after that) is also true for first language acquisition, and that the characteristic deficits for late learners are analogous in sign and spoken language.
Now weirdly, if I remember Pinker right, many of these late first language learners were highly functional people. Many of the subjects were migrant farmers. Also, after learning a first language, many reported having had rich beliefs prior to learning language. I think one reported that when he saw the Pacific Ocean the first time he thought it was God's toilet.
The reason I ask this is that my student Joel Musser is working on Brandom and Okrent, and in some ways his position is closer to Brandom's than Okrent's, and I have no idea what Brandom would say about very highly functional languageless adults.* But first I'm interested if any X-Phiers have started to research these communities or if there is a rich debate on the scientific literature that is there.
Likewise, there is a huge scientific literature on aphaisa, some of which Pinker cites. Again, you get highly functional adults who cannot speak, and (in cases such as recovery from strokes where the aphaisa gets better) who later report having had definite beliefs and desires that they were unable to articulate. As with the case of people never taught a language, is there a tradition of thinking about aphasia in the philosophy of mind, and if so, where should one start?
*I know what he should say (that practical inferences are prior to theoretical ones in roughly the way we learn from Heideggerian philosophy of mind, and the manner in which this holds makes room for languageless thought), but also that he cannot say that for a variety of other reasons.]