In Chapter Twelve of To Kill a Mockingbird, by Harper Lee, Calpurnia is not being hypocritical at all when she speaks in a refined manner with whites and adopts a different way of speaking with some of the people she goes to church with.
When the children ask Calpurnia about this, she tries to make them understand. She explains why she speaks the way she does with her peers:
Well, in the first place I'm black—
In saying this, Calpurnia makes note that she is a member of her black community, and we can infer that she would not want to disrespect them in the way she speaks—she recognizes her place with others of her race...to an extent. She speaks as they do, but she is also proud of how she has learned to speak. She just doesn't walk around boasting about it.
She asks Jem and Scout if they would go home and speak "colored-folks' talk."
...it'd be out of place, wouldn't it? Now what if I talked white-folks' talk at church, and with my neighbors? They'd think I was puttin' on airs to beat Moses.
Knowing her place is not the only thing that drives Calpurnia to act the way she does. The reader learns in the story that Calpurnia has a strong sense of "fair play." It is why she is such a good caregiver for the children. In speaking in an educated fashion at church (for example), she fears it would be taken the wrong way and/or hurt others' feelings.
It's not necessary to tell all you know. It's not ladylike—in the second place, folks don't like to have somebody around knowin' more than they do. It aggravates 'em.
If she were to speak in a more educated form of English with her friends and neighbors, Calpurnia explains that it wouldn't help them to learn to speak differently because "they've got to want to learn themselves." You can't, she says, force people to learn what they have no desire to learn. In this case she decides it is best...
...to keep your mouth shut or talk their language.
It is expected that when Calpurnia works, she sets a good example by which to teach the Finch children how to speak in an educated manner. Calpurnia is educated and has educated her son as well. But she sees more harm coming of throwing what she knows in the face of her peers. To keep peace and avoid hurting feelings, she speaks in a manner comfortable to those she associates with socially or says nothing. She is honest in her explanation to the children. Except for Lulu, no one in her community resents that she works for Atticus; in fact, they have a great deal of respect for him. She is not a hypocrite, but she is also not a snob. She speaks as her peers speak when she is with them, showing respect and regard for their feelings.