One way language functions in To Kill a Mockingbird is the development of the narrator's characterization. Scout is a young child when the novel begins, and the language captures her childhood innocence in some sections. Consider her first encounter with Dill:
I asked Dill where his father was: “You ain’t said anything about him.”
“I haven’t got one.”
“Is he dead?”
“Then if he’s not dead you’ve got one, haven’t you?”
Scout's Southern dialect is captured here as well as her naïveté about the adult world that surrounds her. Since a central theme to the novel is the loss of innocence, language is pivotal in shaping the reader's perception of Scout's character from beginning to end.
Through the language, we also become aware of Scout's perception of the world around her. Sometimes this is quite humorous, such as her dismay with the type of father Atticus is as chapter 10 begins:
Atticus was feeble: he was nearly fifty. When Jem and I asked him why he was so old, he said he got started late, which we felt reflected upon his abilities and manliness. He was much older than the parents of our school contemporaries, and there was nothing Jem or I could say about him when our classmates said, “My father—”
Our father didn’t do anything. He worked in an office, not in a drugstore. Atticus did not drive a dump-truck for the county, he was not the sheriff, he did not farm, work in a garage, or do anything that could possibly arouse the admiration of anyone. Besides that, he wore glasses.
The professions which Scout finds noble enough to be proud of in her early childhood are somewhat humorous in light of the fact that she considers Atticus "feeble" at "nearly fifty." And she doesn't yet realize the significance of the work Atticus does, but this is part of the shift in perspective we gain through the language of the novel.
Language is used to shape the characterization of characters other than Scout, as well. Consider Burris Ewell's reaction to his new teacher's request to sit down on the first day of school:
“Report and be damned to ye! Ain’t no snot-nosed slut of a schoolteacher ever born c’n make me do nothin‘! You ain’t makin’ me go nowhere, missus. You just remember that, you ain’t makin‘ me go nowhere!”
Burris's language spews hatred, much like his father's we find later in the book. Thus, language is also used to reveal some of the ugliest parts of human nature which Scout encounters in Maycomb.