"Then they said to one another, “In truth we are guilty concerning our brother, in that we saw the distress of his soul, when he begged us and we did not listen. That is why this distress has come upon us.” And Reuben answered them, “Did I not tell you not to sin against the boy? But you did not listen. So now there comes a reckoning for his blood.” They did not know that Joseph understood them, for there was an interpreter between them. Then he turned away from them and wept. And he returned to them and spoke to them. And he took Simeon from them and bound him before their eyes. And Joseph gave orders to fill their bags with grain, and to replace every man's money in his sack, and to give them provisions for the journey. This was done for them." Genesis 42:21-25

Just an interesting little thought: other than maybe the occasional interaction with a Hebrew, Joseph had probably not heard Hebrew spoken since he was sold into slavery. Most people who haven't spoken a language for a while will often get rusty with it. They say, 'I can understand what I hear better than I can speak it.' It is amazing to think what emotions must have started to surge in Joseph's heart hearing his own language again. And to hear his brother's speaking of their treachery against him. What an out of the blue experience for Joseph!