Pistol the fool.

How would a poor, probably illiterate foot solider know fluent French.  I would say that Pistol knows some French because he says to his captive in sloppy French, "Yes, cut your throat, by God."  Even though Pistol knows some French he can not understand it because he has no idea what his captive is saying.  At one point in their conversation(if you could call it that) the French soldier says the French word "Bras" which means arm, Pistol misunderstood and thought that the soldier wanted to pay him in brass, he didn't like the sound of that.  Pistol views his captive solely as a way get more money because if the Frenchmen doesn't pay him he is going to kill him.  King Henry on the other hand treats his prisoners as the rules of war require, that is until he sees that the French have killed the English pages and destroyed their supplies.