It’s a very good question but unfortunately the answer is we have no idea at all!

The background to this is that the crusaders captured Antioch from the Turks in June 1098, after an extremely long and difficult siege, and then immediately afterwards, a relief army led by Kerbogha, the atabeg of Mosul, arrived and blockaded them in the city. On June 27, an embassy was sent to Kerbogha. The leader of the embassy was Peter the Hermit, who had preached the crusade in France in 1095-1096, and had participated in the first wave of the crusade, which was destroyed by the Turks as soon as it crossed over into Anatolia. But Peter survived and joined up with the second wave of the crusade.

Peter was considered either important enough to negotiate with Kerbogha, or perhaps expendable enough to be sent into a dangerous negotiation where he might end up being killed…in any case, the crusade chronicles sometimes mention that several other people went with him, and sometimes an interpreter named Herluin is mentioned.

Herluin could speak “their language”, although the chroniclers don’t seem to be quite sure what language that was - Arabic? Turkish? Persian? Presumably the Seljuk Turk Kerbogha spore Turkish, but the Turks were culturally Persianized and often spoke Persian, and Arabic of course was still the common language in the Muslim world. They might not have spoken to Kerbogha directly, but to his own ambassadors and interpreters, who also could have spoken any of these languages.

Kerbogha respected their role as messengers and they weren't killed (the crusaders were apparently worried about this). But of course he refused to surrender and the crusaders likewise refused to surrender. In the end, the crusaders marched out of Antioch and unexpectedly defeated Kerbogha in battle and they were able to consolidate their control over the city and continue marching south to Jerusalem.

So who was Herluin? Well, Herluin is a Frankish name, attested in northern France, and particularly among Normans, such as Herluin of Conteville (William the Conqueror’s step-father) and Herluin, Abbot of Bec. So maybe this Herluin went on crusade with Robert Curthose, Duke of Normandy (and William the Conqueror’s son).

But the name could also have been used by the Normans in southern Italy. In that case Herluin the interpreter could have been a member of Bohemond or Tancred’s army. The Normans in Italy and Sicily

"had been dealing with Arab-speaking Muslims for a generation and had knowledge of Arabic before coming to the East.” (Atiya, pg. 206)

Herluin also could have had experience in the Byzantine Empire, as the Italian Normans often did. Some of them served in the Byzantine army; some of them (like Bohemond and Tancred) also raided the Empire, but one way or another, Herluin could have learned Turkish from the Byzantines.

Of the chroniclers who were present on the crusade, only the Gesta Francorum mentions Herluin by name. The author of the Gesta was an anonymous knight in Bohemond’s army. The other chroniclers who mention Herluin weren’t actually on the crusade and used the Gesta as a source (Baldric of Bourgeuil and Robert the Monk back in France, and William of Tyre in Jerusalem). The other chronicles that mention this embassy (e.g. Albert of Aachen, Fulcher of Chartres) only mention Peter and not Herluin or anyone else. This

“suggests Herluin may have been in Bohemond’s train along with the Gesta author, and the fact that he is named, rather than simply referred to as an interpreter, could suggest noble rank or some other kind of importance, or could reflect the Gesta author’s comfort with multilingual situations and the strategic relevance of interpreters.” (Tuley, pg. 316)

So, all we know about Herluin is that he was an interpreter in Peter the Hermit’s embassy to Kerbogha outside Antioch in June 1098. He knew…a language, but whether it was Turkish, Arabic, or Persian is unclear. Circumstantial evidence suggests he was an Italian Norman, and if so, then he could have had opportunities to learn Arabic in Norman Sicily, or Turkish in the Byzantine Empire.

Sources:

Thomas Asbridge, The First Crusade: A New History (Oxford University Press, 2004)

K.A. Tuley, “A Century of Communication and Acclimatization: Interpreters and Intermediaries in the Kingdom of Jerusalem”, in Albrecht Classen, East Meets West in the Middle Ages and Early Modern Times (De Gruyter, 2013)

Hussein M. Atiya, "Knowledge of Arabic in the Crusader States in the twelfth and thirteenth centuries", in Journal of Medieval History 25 (1999)