By the logic of “this isn’t Earth”, you’re making it sound as if any Earth language would be inappropriate. “Mayhap” and “anyroad” aren’t Old English words. Old English, as I said to you before, is completely unrecognizable from any form of English you would understand. It’s so far derived from Modern English that English speakers have to be taught it like they do a foreign language.
“
Mayhap” is actually from Middle English—the ME word “hap” means “chance” or “fortune”.
“Anyroad” is still in use today in some local dialects (as another poster pointed out above). It may sound strange to those not from the area that it’s used it—but that’s how dialects are. It would be the same if I used my Southeastern American dialect when I wrote posts.
The bad voice acting likely makes it seem like a “badly spoken high school play”—but if the VAs were better, it likely wouldn’t sound that bad. If you understood Japanese while holding your same opinion about the English, you’d likely feel the same way. It’s only because you don’t understand what’s being said in the Japanese audio that you don’t find it strange, in my opinion.
I think the jargon they use can be charming. It’s the bad voice-acting that’s so off-putting to me.