This is one of those things that I leave completely up to the preferences of, well...everyone! 8D
SotE isn't medieval Europe or even our world, so the people there would have developed their own expressions. I know I use some "modern" things, like I've had character say, "Bite me!" or "Wow, that sucks..."
I think I've even had someone say "S'up?" before, lololol. But I guess what I'm saying is that since SotE is set in a completely made up world, hey, who's to say that such "modern" slang isn't current for them, since they would have developed their own languages and stuff differently? Their world isn't a direct reflection of ours, and probably wouldn't develop in the same way, so hey! Sure, some things make less sense, like, say...referencing a tank when there aren't any tanks and other such anachronisms. But I'm not gonna worry over a slip-up! But heck, the characters in SotE aren't even speaking English, or any of the languages on Earth, since those don't exist! We just suspend disbelief.
So I think this is one of those "go by your own preferences" things. If someone wants to use more old-timey language, toootally! But it's not required.
I think people are pretty good about keeping things pretty setting-appropriate from what I've seen, though! But yeah, it's sorta wibbly wobbly and flexible since SotE is in a made-up world loooosely based around a medieval time period with a tooon (so, so many tons) of creative liberties taken. xD