Harper Lee absolutely uses language to enhance characterization in To Kill a Mockingbird. By "language" I assume you mean the way people speak (their dialogue) in this novel. This story takes place in the South, so of course we would expect some southern dialect. We get it from many characters, but others have an entirely different way of speaking. Those whose language stands out include Calpurnia, Atticus, Miss Maudie, Bob Ewell, and Scout.
Calpurnia is a puzzle, because she speaks in two separate dialects. For the most part, we hear Cal speak much like Atticus. Later, we discover that's because they were educated in a similar way and from similar sources--the Bible and law books. When she gets to church, though, we hear an entirely different Calpurnia, one who speaks in the Negro dialect. She makes that change, she explains to Scout, because she doesn't want them to think she's "puttin' on airs." Both ways of speaking suit the two parts of Calpurnia's life.
Atticus speaks in a rather formal langauge, generally much more formal than those around him. That comes from his early training in reading but also from his work in the law. This way of speaking suits him because it sets him apart, somehow, as being an upright and educated man--one who would do what is right no matter the cost.
Miss Maudie is a southern lady, but she speaks her mind. She's not one to gossip, but she's not afraid to raise her voice when the "foot-washin' Baptists come glowering through with their condemning scriptures. A good contrast to Miss Maudie is Miss Stephanie, who is much less formal and mature in her dialogue as well as her subject matter.
Bob Ewell is the least educated and respectful character in this group, and his dialogue reflects that. He's not only rude and rather cocky, but he's also woefully undereducated--as when he mistakes appendicitis for ambidextrous in the courtroom. His dialogue is a perfect reflection of his character.
Scout is another whose language matches her character. She's sometimes irreverent and sometimes a hothead, and her language reflects that--"Cecil Jacobs is a big wet he-en!" When she's more contemplative and serious, her language reflects that, as well, though she is still young and sometimes gets it all wrong (as in the scene outside the jailhouse).
If one were to select some passages of dialogue for each of these characters, a reader could pretty quickly and correctly ascertain what kind of person each one of them is, it seems to me. This novel is an excellent example of dialogue used to create and enhance character.
Without the immigration of foreign-language speakers and new arrivals from Europe in the South, the English that many of the Southerners spoke in the 1930s retained traces of archaic forms whose usage was not as frowned upon as in the North where the evolution of English was more in place. One example is the use of the archaic, and now substandard word ain't as well as the dropping of the g with words ending in -ing. While Atticus Finch has been formally educated, he does not disapprove of his daughter's using the word ain't because in Maycomb it is merely colloquial and not considered substandard as in other areas. Thus, Scout and Jem are allowed to use this word by Atticus because he wants them to fit in with the other children, whereas in other parts of the United States, parents would insist that the children not use such "bad English."
Fitting in with one's social group is extremely important with both blacks and whites. This is why Calpurnia speaks standard English around the Finches, but reverts to a Negro dialect when she is among her contemporaries; she does not wish to become alienated from either group.
Certainly, social class is indicated by the level of language that one employs. Interestingly, when Mr. Raymond Dolphus speaks to Jem, Dill, and Scout, and it is not just the "only English riding boots I had ever seen" that Mr. Dolphus Raymond wears, but his well phrased--abeit colloquial at times--Englsh that distinguishes him from others in Maycomb. In his keen perception, Mr. Raymond recognizes that Dill will not cry in a few years. When Scout asks him "Cry about what, Mr. Raymond," he replies with beautiful parallelism,
Cry about the simple hell people give other people--without even thinking. Cry about the hell white people give colored folks, without even stopping to think that they are people, too.
Both Miss Maudie and Mrs. DuBose speak with few grammatical errors--if any--in their language, a fact that connotes their having been well raised by people of social position, as well.
Of course, the gutteral, substandard language that the Ewells speak denotes their lack of education and upbringing. Even some of the vocabulary is incomprehensible to others such as Atticus and the members of the jury, such as saying that Tom had his fingers around Mayella's gullet. Of course, he uses archaic words such as fetch, ain't and crude similes, such as "screamin' like a stuck hog inside the house" and "screamin' fit to beat Jesus," and "that black ---yonder ruttin' on my Mayella," he drops many of the final cosonants of words.