Throughout antiquity and he middle ages, slaves were often used as interpreters. A person taken as a slave fairly young from a part of the world where a language was spoken, brought to live in a part of the world where a different language was spoken, could become bilingual enough to be able to interpret for commercial or political meetings. In the specific case of the crusades, especially for the later crusades, we need to remember that the Latin crusader states had constant contacts with their Saracen neighbors during times of peace, and generations of residents grew up in a land where Arabic was used essentially on a daily basis. For the crusade of Louis IX, for example, we have two names of interpreters listed in the Chronicles of Joinville, Bandouin (Baudoin) d'Ibelin and Nicole d'Acre, both of which can be assumed to be from the Latin crusader states, based on their names.

Source: Jean Delisle, Judith Woodsworth, eds, Translators through history, 2012