Explain what Scout finds unusual about Calpurnia's manner of speaking at the Negro church?

The passages you are looking for are in chapter 12, when Cal brings Jem and Scout to her church.  When Calpurnia is at their house, she speaks "normal", or at least, like other white southerners in the city of Maycomb.  But when they all get to the church, and Calpurnia is around other black people, Cal starts speaking the Negro dialect again.  This startles Scout, and opens her up to the fact that Cal might be more than just their housekeeper; she might actually live a real life outside of when Scout sees her.  Scout thinks to herself,

"That Calurnia led a modest double life never dawned on me.  THe idea that she had a separate existence outside our household was a novel one, to say nothing of her having command of two languages."

So, Scout is impressed that Cal has a life outside their home, AND that she can speak 2 langauges so well.  It isn't really two languages, but she feels like it might as well be.  At the church, Cal slipped back into the Negro dialect, and Scout found it unusual that she did so, but it led to a rounding out of Cal's character for her.