Marcus noted that Esca knew the Northern language and could interpret. Esca was the son of a Brigantes chief and spoke Brythonic, which is close to Welsh. The Brigantes had controlled a large section of northern England. When they pass through Hadrian's Wall into Scotland, Esca speaks in Gaelic to everyone. There were no Gaelic speakers known to be in Scotland in 120 A.D. Most scholars believe the inhabitants [the Picts] spoke a Brythonic language related to but distinct from British. The first Roman recording of Gaelic speakers [the Scotti] in Scotland was in a skirmish at Hadrian's Wall in 297, and immigration wasn't until circa 400 A.D. Esca could indeed interpret, but in Brythonic not Gaelic.
When Marcus and Esca pass Hadrian's Wall and head north, they are both pictured riding bay horses. Shortly thereafter, and for the rest of the film, they are seen mounted on war horses, 1 black, 1 white. No explanation of where they got these horses is forthcoming. (In the shooting script it is stated that they took these horses from the "rogue warriors" they killed, but in the actual film, the horses appear before the scene where they kill the rogue warriors.)