People say Frederick II might have converted to Islam, I am not one but it's muddy, but he did know Arabic and the leaders he engaged to hand over Jerusalem without a fight were impressed by his knowledge of both Islam and Arab ways that he gained from his childhood in Sicily. There was some issue when later crusaders arrived in the crusader states that they had "gone native", adopting the clothing and manners of the very hot near east rather than dying of heat stroke in traditional Frankish clothing.