As an aside, I see no point in not using it when done properly. It just bothers me to see people who can\'t speak without it and attempt to be In Character while using modern English.
I must say I don't get it. Language must be modern English, or it would be impossible to play. Of course you should avoid references to any "modern" object or concept, but that's another matter. While IC you should avoid references to anything that's outside the universe, be it modern or ancient... Think any PS char would ever know anything about dinosaurs? Or egyptian mummies?
Strictly speaking, any kind of English (ancient or modern) is wrong inside the PS world, since there's no England (or America) there. I could use Italian while IC, and it would be just as wrong,
The language is something we (the players, not the characters) agreed upon. It's totally an OOC issue.
Of course if you're good at it, you may try and give your character a trait by consistently using a certain
style or accent. If you're playing a nobleman, you'll speak differently from the local bartender. I'm not a native speaker, and very likely will never be able to use a "style". I'll have to stick to plain English.