To call one's bluff is to demand proof or challenge someone to follow through. Your friend knows how to call one's bluff if she insists you speak in Arabic, a language in which you pretend to be fluent.
If your brother threatens to throw your laptop out the window after finding it on his desk yet again, you can call his bluff by saying, "Go ahead, do it!" You're calling his bluff when you challenge his claim that he'll really do it. This phrase comes from cards — to bluff in poker is to pretend to have a good hand when you don't, and to call someone's bluff is to challenge this by betting high or not folding.