Of course the inhabitants of [Insert Fantasy World Here] probably wouldn't speak English. Of course they wouldn't know common words like "sword" or "dragon" any more than they would "dinosaur." The idea here is that we're using English as a substitute for their Common language. Thus the change of "dinosaur" to "behemoth" favors internal consistency. This is a world where people live in fear of monsters and unknowable evils (at least if one follows the "Points of Light" paradigm). Latinate terms for creatures automatically evokes science and knowledge and taxonomy. The inhabitants of this world know very little of what lurks beyond their respective villages, and much less do they have a sophisticated system of categorization for them.