It's an interesting question. If you want "historical accuracy", you would have the actors speak in Latin or Greek, but these days I'm not sure it'd find an audience. The only place you're likely to find spoken Latin is ecclesiasticly.

So your show's in English but not set in England, what accent should you have? For quite a while, I'd have said "what native speakers from that area sound like when trying to speak English"...

But when I watched the HBO Chernobyl mini-series, they had a different approach. All the various Russian, Ukrainian, etc. accents were instead regional English accents. And that was pretty good, because, honestly, as someone who doesn't speak any Slavic languages, I can't easily tell the differences between them, and I'd miss the implicit cultural and social judgements the speakers make of each other. But I know exactly how an RP speaker looks down on an Estuary English speaker who in turn looks down on a Yorkshireman or Scotsman. By using my own country's accents, the narcissism of small differences comes through loud and clear.

The accompanying podcast said they did it to make the actors "act their roles and not their accents". But it is a balancing act - do you want the versimilitude of having people sound like they're from the place on-screen (while still speaking English)? Or do you want the nuances of language translated properly for you?

In purely fantasy worlds, there's no reason why elves speak RP, dwarves speak Scots and orcs speak Cockney... if anything, the dwarves should speak Old Norse!

On a final note, watch the British take the piss out of those self-serving Hollywood movies where they have Americans play the heroes despite their country not existing until several centuries later (and of course giving the baddies British accents): Jerry Brickhammer presents "The Crusades", the movie in honour of all the Americans who died in the early middle ages: https://www.youtube.com/watch?v=IbzlUQwaC4Q