Every now and then I see an article about language in historical novels, and can't resist reading them. Lindsey Davis says we can be pretty sure that people in 1st-century Rome didn't speak like one of Cicero's speeches when they were talking to their friends, arguing with their landlords or chatting up a girl. Equally I'm pretty sure people in Anglo-Saxon England didn't talk over the morning pottage as if they were reciting Beowulf.
Formal records and poems avoided slang and colloquialisms back then just as they do today - but that doesn't mean they didn't exist. Some authors think colloquial modern English gives a much better 'feel' for the people and their world than the stilted dialogue you sometimes see in historical fiction. I'm somewhere in the middle on this. I like a "timeless" feel if that is possible. I think it can be achieved by avoiding modern slang and using the odd phrase that has a ring of the time about it, as long as it is not overdone, can add the "historical" feel.
The same can be said about dialect. If American novels get by writing in what I call modern "New Yorkese" then what's wrong with me adding a touch of my local Northumbrian or Geordie to the pot? I don't understand many of the "New Yorkese" references (I still have no idea what Nora Roberts is thinking of when she writes of a character wearing a watch cap) but I try and ensure that readers can understand the dialect phrases I use - and I try not to use too many!
Tried the new tv historical Versailles last week, but didn't like it. Lots of little niggles - actors I did not know, no attempt via the dialogue or its delivery to indicate it was all happening in France - lots of characters who looked very similar. (OK, I'll allow that brothers often do look like each other). All that hair was such a turn-off - I kept thinking of head lice and leaning away from the tv set. Raging venereal diseases was very likely judging by the amount of sex portrayed in almost every possible permutation, in the first half hour of the programme. Plus which, not much actual story. A pity. Must see what Kate Williams has to say about it - http://www.bbc.co.uk/programmes/b07ff99m