I have no problem with people defending the things they're passionate about.
I'm just not the sort of person who writes books in order to write a story. I don't outline, I don't pre-plot, I don't draw maps, I don't do character sketches, I don't do chapter summaries. If I'm gonna do all that, why on earth would I then go on to write the story? It's already written, albeit in a non-linear form. I understand for some people "all that" is part of the fun of the craft. For me, it's the kiss of death, and will satisfy any desire I have to tell the story so it would never get written in a narrative format.
I asked because I was wondering how other people would convey the difference. What bits of grammatical finesse I might employ that would get across that there's a gap between the way he speaks and the way she does, and best case do so in a way that the dialogue itself tells you he's speaking an older form of the same language she is. Sprinkling his speech with "thee" and "thou" and throwing in random "-eth" suffixes isn't the answer. I like the use of idiom, though that'll require a deft touch. Pulling it off for the first twenty or so pages should be fun. And I might use more formal English when he speaks.
The use of magic to solve the problem will, however, be a major plot point. He uses magic (of a sort). No one else does, could, or would. It's why he's being chased, and will be the central conflict of the story. He wants to restore the world to what it was. And the world-that-is took a lot of effort to create; it's not going to want to change and possibly it shouldn't. Change is rarely peaceful.
I'm sorry you're peeved I declined the advice to do a lot of research and map out the geopolitical history of the fantasy world in order to answer this question. Or maybe you're peeved at the way I declined it. I wasn't clear enough in the question, I suppose, or my amusement at the answer might be more understandable. It's akin to being asked, "So, in this very first bit, my characters are on horseback though they're mainly going to be in one place through the rest of the book. Still, even though it's a momentary hiccup, I was wondering how you'd convey that they're riding at a fast pace while talking?" and then getting an answer back telling you to first decide how the past 3,000 years have shaped the horse breeds in your fantasy world. It made me smile.
I'd never look for validation about whether an idea is cool or not. Story ideas are like Schrodinger's Cat: they're both at the same time as long as they're just sitting in a box. You have to open the box, write them down, before they'll find a fate.