Yoshi p?
It could be an approximation. I'm English and I'd have said historically accurate rather than something like authentic or so our world building reflects the period of history we would like it to before i gave the wording some thought and even then i'm not sure these statements fully put across what i want to say.
Despite the modern, developed, world being a mixing pot of many different ethnicities and cultures from literally all over the world a century now this has not been the case for many thousands of years. Frankly I'm in awe that coins which date back to and would have been brought to the city by the Norse who had claimed the city something like 800 or 900 years ago have been found in York from as far away as modern day Uzbekistan. Do you have any idea how long a journey like that would have taken using the technology available at the time? How difficult that kind of journey would have been? Even if it was done in relatively short stints. an owner of the coin travelling only so far before trading it to someone else who travelled with it a distance before they traded it and so on the fact something from that far away got into the hands of people that didn't have air travel let alone motorised vehicles or even steam power is astounding and with that in perspective it highlights how lucky we are to live in the here and now with technology that has made the world seem so small and manageable.
It's easy to forget that i'm talking about a game developed by a company based (and i googled this) 5,845 miles from the place i'm writing this and we are just holding a conversation with people from near and far as if we were literal neighbours about weather or not someone from this company that is likely many thousands of miles away from either of us is making bullshit claims about historical accuracy.
The fact is there was a period of time where travel from one end of a relatively small country to another would have taken days or weeks and so what would this mean for international travel?
authenticity could be seen as a sliding scale and while you could build a world that's just as diverse as much of the modern, developed world (though I'm sure there are places with little immigration that would certainly be considered developed however i don't want to go looking up statistics right now) doing so would definitely place it lower on that scale than a world that did not.
I would also add that authentic does not mean historically accurate. The story of Beowulf isn't a historically accurate document unless you believe someone really did fight a giant monster or a dragon but there is authenticity in its setting. if that makes any sense. You could do a modern retelling, turn the meadhall into a Wetherspoons and it wouldn't not be a story of Beowulf but i think it'd be lower on the scale of authenticity than the story scholars have studied but it'd certainly be more authentic than if the story was retold in its ancient setting with the addition that the meadhall was a totally modern Weatherspoons and it's going to raise further questions if the rest of the story is trying to put across the feeling that the story is set in this ancient time and place. Assuming the meadhall is located in denmark or somewhere equally northern and mainland Europe what's a modern English pub franchise doing there? I'm not saying someone couldn't or shouldn't use this as a plotpoint for a story but for something as out of place for the setting it ought to be explained, right? perhaps it could become a new focus point for the story. I can't help but feel this episode of dr who is starting to write itself.
likewise for medieval/dark age Europe the assumptions of authenticity are going to be that travel is limited, difficult and that because of this the term far off land probably means the next couple of kingdoms over at most and since location and ethnicity or race have gone hand in hand for thousands of years prior, right up until this tiny snippet of history that we live in. Again i'm not even saying that there weren't some people who travelled great distances. I said before I'm in awe of the fact that items and perhaps even people could travel from as far as Uzbekistan and i recently read online somewhere that nearby people in the Mediterranean and Africa certainly had contact, though that should be obvious given the wars Rome had in northern Africa. Rome's ability to claim such a massive chunk of land is also in interesting feat given the difficulties in travel I've already written about but if i go off on any more tangents i'll never actually post this comment.
Long story short is that people a long time ago came from places and this was in part surely down to how difficult travel was for them. you can tell a story that draws inspiration from such a period of time which also throws in some more modern expectations but it would be less authentic than a story that does not. This is not a bad thing but like a modern pub in an ancient setting it would require extra leg work to explain and might even need to become a focal point of the story which might not be what the writer wants to go for.