In terms of the language used in Harper Lee's To Kill a Mockingbird, we hear the "voice" of a young Scout narrating the story as an adult. We are, however, able (through Harper Lee's gift of writing) to hear the child Scout was, come through in the pages of the story, capturing the innocence, concerns, joys and fears of a young girl.
Another aspect of the language, perhaps more importantly, is the dialect used. The story takes place in Alabama, in the imaginary town of Maycomb. The town is in the middle of the Great Depression, and the end of the Civil War is not so long ago that people have forgotten how the South suffered; many of the townspeople feel a strong resentment for the non-slave black community which still suffers deeply from prejudice at the hands of these whites. Many of the members of this small-town white community still speak the language of the Old South—where education and cultural growth have not arrived with enough force to change their language—so different from the language of the North.
Additionally, among the whites there are delineations. There is language of people like Atticus and Miss Maudie, and then the language of the Ewells. Importantly, there is also the language Calpurnia uses with the Finches, and the language she reverts to when among her own people.
If the language can have any symbolism, I would expect that it shows clearly the divide between the educated community and the uneducated community. Most of the time, people like Atticus, Miss Maudie, Aunt Alexandra, Judge Taylor, Uncle Jack, etc., show their standing in the community by the way they speak: they are more sophisticated which would symbolize their intellect, education, and their ability to rise above the circumstances that keep the rest of this this town from moving forward more quickly into the newly- arrived twentieth century. This learning is seen in Calpurnia's speech and ability to read and write. She is a character that walks between the two worlds.
On the other hand, people like Bob Ewell are a clear example of this part of the country and its inability to move beyond its past and its sense of being persecuted and treated unfairly in the war between the North and South. Ewell, his son Burris, and daughter Mayella show through their speech how uneducated—and hateful and suspicious—they are, still mired down by the sensibilities and prejudices of the Old South.
For example, when Mayella takes the stand in court, she becomes extremely argumentative because she thinks that Atticus is making fun of her. It is only when judge Taylor assures her that Atticus is being polite that she will answer his questions.
She was looking at [Atticus] furiously.
"Won't answer a word you say long as you keep on mockin' me…
And to Judge Taylor she declares:
Long's he keeps on callin' me ma'am an sayin' Miss Mayella. I don't have to take his sass…"
Even Bob Ewell's language shows his sense of the "good old boys" of the South when he acts without respect toward the court and its representatives. When asked if Mayella is his daughter, his response is:
Well if I ain't I can't do nothing about it now, her ma's dead...
Just before sundown. Well, I was sayin' Mayella was screamin' fit to beat Jesus—
I believe that the different types of language used in the novel symbolize the educated from the uneducated, the forward-thinking from those locked in the South's past.