Yeah, in a way it's true about the Imoen accent. The game as such "feels" ancient, in some old old time and then this Imoen comes out and speaks in the dang 'modern' language heheh.. American, not american, who cares. Her spelling felt 'out of the world' in BG
Take in the first dungeon, the Irenicus dungeon: "I cannot believe that this room belongs to the same person who owns those horrible glass-containers" - dang, how she spells it, especally the 'glass-containers' part.
I am from Europe, but not from UK. So I aint speaking from that point of view. Simply the Imoen's spelling sure is too "modern" for the game setting. IMO.
I aint telling though that the Jaheira and Minsc have the 'european' accents either. Heck there's no such country in BG
Let's just say that their accents are the accents of those who may not speak the 'common', human language as their main language.