Once again, they were speaking draconic. He didn't know a word of what they were saying, only that people who had been fighting a minute ago were speaking a strange language at each other. In a world where speaking in a strange language is a common prelude to a fireball coming out of midair this isn't always the best sign, especially for a character without the "knowledge" of a monster's capabilities, or a decent spellcraft.
If you're going to ask that a wyvern not attack an armored troop of individuals walking through the forest, which is a common prelude to a fireball coming out of midair, I think we can ask that a paladin not attack a creature his party is talking to, in any language.
In any case, that's irrelevant. If the paladin attacks a creature his party is talking to, ignorant or not, if that's known, other creatures will be hesitant to parley.