To Kill a Mockingbird- Chapter 12

'They's my comp'ny,'

‘They’s my comp’ny,’ said Calpurnia. Again I thought her voice strange: she was talking like the rest of them.

Calpurnia has excellent English but when she is around other black people she adoptes their way of speaking. She does this so that they don't judge her as following the 'white people' , and they will treat her with respect as she is an equal. Calpurnia is knowledgeable and is an intelligent women, she can read and write, and taught her own children to do so as well. This was how she picked up the language.