In "To Kill a Mockingbird" why does Calpurnia speak two languages?
Calpurnia doesn't speak two different languages, exactly, but she does speak two different versions of the same language depending on her audience. A term for this phenomenon is "code-switching," which occurs when a speaker uses different manners of speaking at the same time in order to make him or herself understood.
Calpurnia speaks in grammatically correct English with the Finch family, and then when she returns to her own community, she speaks the way they speak, which means she is less concerned with the rules of proper spoken English. Calpurnia can code-switch easily between the two groups of people, and she does so to facilitate communication and to make sure that her connection with her community is intact despite her working for a white family.
When Calpurnia is with the Finch family, she speaks a version of English that white people find acceptable, but when she goes to church with her own people, she speaks as they do. Calpurnia does not want to experience rejection by her employers or her community, which could happen if she did not code-switch. For instance, if Calpurnia's people heard her speaking to them as if they were white, they might feel that she is looking down on them. Potentially because Calpurnia wants to avoid this conflict, she code-switches, as it enables her to protect her different relationships.
When Calpurnia is asked why she speaks like other people of color when she attends church, her explanation is that she does not want her fellow congregation members thinking that she is condescending to them.
She even tries to frame her answer in a way that Scout can understand, by asking her if she would feel "right" about using a different dialect in the presence of her family and friends. By putting Scout into her own (Calpurnia's) shoes, Cal helps Scout to understand why her "double life" is necessary.
The irony is that Calpurnia is capable of not only speaking correct, unaccented English, but she is also one of the few church members who can read and write, by her own admission. Calpurnia's ability to slip between the worlds of white and black southerners is a bit distressing to the children at first, but once Cal calmly explains its necessity, the scene is one of the more memorable in this novel.
One "language" is her formal speaking and one is her casual speaking voice. Think about how you talk to your close friends (or text) and then think about how you would talk to your parent's boss--one is more formal and the other is less. She doesn't technically speak two languages--they're both English.