In Harper Lee's novel, To Kill a Mockingbird, even though Lee is an author, I do not believe that she would agree (in the context of the story) that illiteracy is an insurmountable barrier to communication.
Communication is a funny thing. It can take place with words in some situations, and in others, no words are necessary. For example, look at two people who are in love, or a mother with her baby. Sometimes words are necessary to convey sophisticated ideas, laws, thoughts, and beliefs, but sometimes "actions do speak louder than words."
For example, in Scout's classroom, when she first goes to school, no one else can read (as far as we know), but the children have no problem communicating with Scout. There are certainly members of the black community that cannot read or write. In fact, Calpurnia says that she is an exception because when she was younger, she was taught by "Miss Maudie Atkinson's aunt, old Miss Buford…" Calpurnia, in turn, taught her son Zeebo to read as well. However, the rest of the congregation cannot read, so when they sing hymns, they do something called "lining," which means the song leader (Zeebo) sings a line, and then the congregation echoes, or repeats it, back to him. However, these people do not struggle trying to communicate.
Communication is not simply a question of reading and writing above all, but speaking and listening—and showing your intentions by the way you act.