Why does Cal speak differently at First Purchase?
There is an interesting irony when Calpurnia takes the children with her to the First Purchase African Methodist Episcopal Church in To Kill a Mockingbird: Some of them question Cal about why she is bringing white children to a Negro church,
"What you up to Miss Cal?....
"They's my comp'ny," said Calpurnia. Again I though her voice strange: she was taling like the rest of them.
After the church service, Scout asks Calpurnia several questions and deduces that Calpurnia leads "a double life." When she inquires as to why Calpurnia has spoken in the manner of the others, Cal tells her that first of all she is black, but she also explains that to speak as she does in the Finch home would be out of place in the community of her church, and the church members would find her pretentious:
"They'd think I was puttin' on airs to beat Moses."
Certainly, the visit to Calpurnia's church has been a true learning experience for Scout and Jem.
Simply, Calpurnia is using language that's appropriate to her audience, and to the situation, a language that binds her to her community rather than separates her from it, as was said in the previous answer. As part of the theme of the novel, it's another chance for Scout's world to open out, to walk another mile in someone else's shoes, to see that her world intersects with others, like the Ewells', and that the worlds aren't all as safe and comfortable as hers. This is an important part of moving from self-centered childhood into a more adult understanding of society and the people in it, even if it results in the loss of childhood idealism and "innocence." In providing an opportunity for Scout to see this in action in the "double-life" led by her beloved housekeeper, Lee gives an illustration that the child Scout can understand, and that the adult Scout (the book's narrator) - and the reader - can see as a significant part of the learning experience.