Marc de Hauteville was his baptismal name, Marc for the a Roman god of war. " Bohemond" given to him by his father as a nickname of a legendary giant, because of his size. Most of his life he signs his name as Marc, not until much later in his life is he Prince Bohemond. His mother was Lombard and his father was Norman, so he most likely spoke medieval French and German, but not necessarily Italian and so his real French names are probably closer to reality. Like the upper classes of Normans in England, they would have spoken French, not the language of the people's they conquered. He is only the second generation of Normans in Italy, so they are still speaking their own language. He was a self crowned Prince after taking Antioch in the first Crusade, not a prince of the blood. In Italy, he was given a Principality in a settlement after his father's death, Tarato. It was his half brother, a Roger a Borsa, who became Prince of Apulia.
Lots of good links here for original resources:
http://distractingfromthenow.blogspot.com/2008/06/bohemond-of-taran...