Answer:
Calpurnia says that she talks differently to the people at her church because it would be out of place to speak the way she does when she's around Jem, Scout and Atticus. Calpurnia says "...folks don't like to have somebody around knowin' more than they do. It aggravates 'em...when they don't want to learn, there's nothing you can do but keep your mouth shut or talk their language" (Lee, 127). Remember that Calpurnia speaks better English than those at her church, so matching their speech is important for communication.
Reference for this can be found on Line 13 of Page 127