This is probably not confined to historical writing, and is related to Puma's comment.
My betas have told me that they
can't stand accents in writing. (I can't either, but I wanted to check to see if it was just my pet peeve)
For example, JK Rowling's character Fleur, who is French, would say things like "But zis ees so wonderful, ees eet not?" Ugh.
So if writers are tackling multiple nationalities, I think it's enough for the characters to throw in a native word or two. ("This is wondeful news,
non?" "Bad luck,
mein Herr. The House wins again")
Also (and this has been a discussion on another board) it will immediately undermine any historical story if
the author uses modern character names. But - and I suspect this applies especially to Regency Romance - authors seem determined to call their MCs names which were unheard of before 1950 (sometimes before 1990!).
Another thing that has stumped me is the question:
what do characters do for a living? Not every lower class woman is a seamstress or a prostitute; not every mddle class man is a barrister or a bank clerk. And adding in an interesting, lesser known trade or profession (say, hod carrier or match-maker) will add depth to the novel in a way that a thousand egg-and-milk women can't.
My only suggested solution is that, whenever the writer is reading a book from her era, she makes a note of all the professions and trades mentioned. She can research them later and decide if they're right for her characters.
Think of it as being a careers counsellor for your MCs
Well, that's all I can think of... Hope it helps!