Oh I totally get it -- sage brings to mind sage brush which makes you think of desert Westerns which aren't medieval. Which is why writers can't worry about stuff like that. Readers can get pulled out for all sorts of reasons. Mostly, it's because they're not that interested in the particular story, so the brain picks on other things.

If you have a story set in a sec world that is like ancient Rome and the character goes "Dude, you are so gangsta" that's not necessarily a problem. In comic fantasy, the contrast of a period that resembles Earth contrasted with modern language is often a deliberate part of the humor. Another use is to give a story a certain tone -- the characters in a medieval world talk like they're Vietnam War soldiers (Black Company,) etc. on purpose to give the characters, the world and the narrative a certain sound. Because it's not Earth. Writers doing secondary worlds blend cultures and things from different time periods. You can literally not have an anachronism in a secondary world. Anything is kosher. Whether it will be engaging to you is another issue.

That being said, a database of medieval words and terms can only be useful. Even if you're writing a contemporary fantasy, it could be useful as medieval issues may come up. So I think a database is a great idea -- it always amazes me what people here know, like say the origins of the word damask -- but there is a lot of research material already out there on the Web and through libraries. Everything is fodder in fiction, especially SFFH.