Translation Team: Please stop doing this...

[kyuven]

I'm no expert on the specifics of Japanese dialect, however:
It can change things a bit. In the ARR quest where you meet Nanamo in disguise she uses ''watakushi'' which implies she is noble. (despite the fairly low grade garment she is wearing) The dialect she uses kind of gives her away, as it's a ''formal/old''ish- dialect. Because of that its fairly obvious that it indeed is Nanamo in disguise.
The dialect she uses is very much a ''highborn'' dialect. Using a different dialect for her would portray her as a different person, I think.

"Watakushi" doesn't really imply "noble," more like "well-bred." A prominent merchant's daughter would use it as well (such as, say, Lolorito's female family members)
"Wareware" is what they use in "noble" situations, and it's kind of a "royal we", though not limited to nobles in the slightest (Merlwyb, Kan-E, and Raubahn all use it, and it's usually use to imply "we as a group/nation/whatever")
Japanese is very complicated, but everything used in-game with the exception of Urianger, Ramuh, etc. is pretty bog standard Japanese speech still used today. It's entirely possible to run into someone who speaks Japanese exactly like Nanamo in the real world, but night impossible to meet someone who speaks English like her.

Good point, but you just made me think of some funny examples. If the guy says "konichiwa" don't translate that as "Salutations and greetings, honored friend!" And I think that's the only real problem I have with the common speech in this game, sometimes a modern Japanese phrase is turned into old fashion fantasy English gibberish for no reason.

Sometimes, admittedly, it has to do with mouth flap matching. Kon ni chi wa has more mouth flaps that "hello" or "greetings..."
Though they don't always exert the effort to match mouth flaps, so this may be a digression.