There is a delicate balance that must be maintained when writing "historical" dialogue. Yes, people spoke differently in former times, but unless the writer is an expert in that period's idioms and expressions, "historical" dialogue should not be attempted.
In other words, if you're not Shakespeare, don't try to write like Shakespeare.
However, this does not mean your characters who are wearing the hoop-skirts of the 1860's should speak like modern teens on Twitter. (Lik3 will u waltz w/m3??)
Example: the dreadful dialogue between Snow White and Prince Charming in the pilot of Once Upon a Time. Don't get me wrong. I love Once Upon a Time. But there was some truly clunky dialogue in that episode.
Charming: Snow, we can't keep having this conversation. She wants to get inside your head. We're about to have a baby.
Snow White: She poisoned an apple because she thought I was prettier than her. You have no idea of what she's capable.
Allow me to nit-pick this bit of dialogue. First problem: "We can't keep having this conversation" is a completely modern expression. Second problem. "...she thought I was prettier than her." It ought to be "prettier than she [is]." Third problem. "Of what she's capable?" Is Snow White suddenly channeling Yoda?
This scene in "Once Upon a Time" is set in a Medieval-ish fantasy world. I am by no means an expert on Medieval speech patterns, so I'll re-write the dialogue to remove the modernisms and the truly awful Yoda-ish mess, without lapsing into fake Medievalism.
Charming: I've told you again and again, the curse was an empty threat. When our baby is born--
Snow White: The Queen doesn't make empty threats! She poisoned an apple. She hunted me through the forest. She wanted me to die because a mirror told her I was more beautiful than she was! What do you think she'll do to my child?
And then my Snow White would throw something at Prince Charming being such an idiot as to imagine that a homicidal jealous witch-queen with unspeakable powers WOULDN'T mean it when she said she was going to curse the world.