Yes, exactly that. It's a fantasy setting therefore the usual elements are accepted. However, using language taken from from example, 21st Century US TV shows isn't believable. In the same way that if a modern Neil Gaiman story with fantasy elements was dramatized for TV, it wouldn't be believable if the carachters spoke in Olde English.
Being set in an historically accurate Medieval environment is not essential to the Fantasy genre, though, any more than aving to speake wit yon tonge of ye Middel Englyshe, as evidenced in modern Shakespearean interpretations.
In fact, using modern culture to reflect the period equivalents and help the modern audience identify is a pretty clever and fairly common filming trick, used to very good effect in some other productions such as A Knight's Tale (2001) or Peter Jackson's series of Tolkien films.
As is, there are a few things that don't fully translate from the original Slavic, such as the Law of Surprise, to a Western audience, so is it any wonder that a few things seem slightly 'off'?
Moreover, this is on Netflix. It's intended to reach a far wider audience than just those few screaming fanboys who demand a 100% completely accurate documentary copy of the books.
But since this is also a made-up fantasy setting, people can talk however the **** they like, because IT'S NOT REAL... Hell, it's not even historical fiction like something penned by Cornwell, so no need to burden people with historically accurate bull ****. Even common speech of the 1800s would be unrecognisible to us today - You're already having trouble keeping track of fairly straightforward split timelines, so how would you cope with having to learn what is effectively a completely different language on top of all that?
As for the acting, I noticed a lot more subtlety than you'd usually find in today's modern spoon-fed drama, with all it's high-octane tension... and just because the plot isn't all laid out and explained to you every step of the way doesn't mean it's no good. You seem to have read the books, so you'll know the real reasons for the "cringey, boring, inevitable love story".
And you can **** off with criticising the bard - Being such an authority on the fantasy genre, you'l know that bards are
meant to be utterly terrible!! Jaskier is absolutely the embodiment of the bardic tradition, while at the same time perfectly parodying it. The signature song with its awful lyrics doesn't take itself seriously at all, yet is already one of the most memorable bard songs in the whole genre!