he doesn't use medieval spellings, but he does use words that are antiquated - not that they've been in disuse for centuries, but for a good while at least even at the time he wrote the series. A good example is his regular use of "ne ... point" instead of "ne ... pas" - I'm not really quite sure when that went out of fashion in the spoken language, but even when he wrote the books, it was already said only by people who wanted to sound intentionally old-fashioned. There are other regularly recurring examples, such as "courroux" for "anger", "savoir (qqn) gré" for "being grateful", "davantage" for "more", "bru" for "daughter-in-law" and so on (some of those might be used on occasion still even in the spoken language, but they feel rather old-fashioned to me at least). I imagine you'll have encountered most of those at some point or other in your readings already. I should imagine your main problem with the language will be the same one I had - highly specific medieval vocabulary, names of weapons, tools, that sort of thing.