Most people would have been wary of a Tevinter lord. Antonio merely takes in the other man's appearance and accent and does nothing. The perpetual slapfights on Seheron made Tevinter a valuable customer for his wares so the man's presence was of note only because of how far from the North they were.

"Don't tell me they all think they're good at their jobs." Antonio sticks with Common, presumably to avoid hearing broken Antivan. He could have switched to Tevene, he's done enough business with them that it was worth learning the language, but it would draw the wrong kind of attention.