To be blunt, I think your opinions, OP, are being heavily influenced by the fact that you're watching the cutscenes in a fashion that was not intended - reading one language while hearing another. The localisations are all different for a reason, I suspect it being something to do with cultural differences, so in your head there's a culture clash that wouldn't happen if, you know, you just picked a language =P (Don't eat me, I was joking, play it the way you want to)

You're also willfully ignoring that pepole have given you an answer to your question as to why the language is the way it is in English. It is a thematic choice taken because the setting for Eorzea and the tropes surrounding the story are heavily drawn from a time when such language was common. Does a similar manner of speaking exist in Japanese/French/German? Probably. However because there's no correlation between those languages and the themes running through the game, there's no reason for them to use the language.

I heavily suspect the sentence you used as an example in your OP doesn't exist, and if it does I'd love proof, as I've never noticed an example of poor grammar in the entire storyline. Just because they follow old usage doesn't mean it's wrong, nor is it another language. It also shouldn't be hard to understand for an adult who has English as a native language, as very few words they use are particularly obscure, just used differently to how we use them today.