I did ponder this question when I wrote a paranormal romance where one of the MCs is an ~2500 year-old fae born in Campania.
I chose him to have an accent, but not one easily placed. But he's smart enough to become fluent in English. And he's old enough to have become fluent in English as it changed. So he speaks modern English well. He does tend to have an expanded vocabulary and will use less commonly used words if they're the one he wants (and at one point, gets teased about his word choices by his lover).
But he'll also use contractions, 'cause damn, the Romans did, too.
I suppose it
does depend on when they learned English and how long they've been speaking it. But sheesh. If you're an old vampire and it's the 21st century, you've probably been speaking English for
at least 100 years. Get over the contraction thing, already.
But yes, some ancient beings won't be speaking at all, will they?
I suppose it's just a buggaboo of mine. People's accents do soften over time. And they do get better at language. I just can't see intelligent immortals getting hung up on contractions. There are other ways to give them a different voice.