And they are VERY different from a 21st century French noblewoman.
When writing details of the time period, an author could research the different foods a character is eating (Pro tip: McDonald’s weren’t period!), clothing, the way they made their living, etc.
Speech and idiom is the hardest part. It’s a balancing act. Of course the 12th Century Irish character isn’t speaking anything resembling English. They aren’t even speaking modern Irish. They’re speaking middle Irish, and no one today outside of a few scholars would easily be able to read it. I certainly wouldn’t be able to write it. Even if it was in England, 12th century language is very different from today’s. If you doubt me, go read some Anglo-Norman works. English as a language didn’t exist – it was a proto-mix of German from the Anglo-Saxon peasants and French from the Norman nobles.
So we use mostly modern English in historical novels. But we can’t use pure modern English, as that would sound strange. Telling someone that the assassin was going to ‘pop a cap’ in his victim’s head just seems… wrong.
Most historical fiction authors sprinkle older words and phrases into modern English and try to limit the anachronisms to give a ‘flavor’ of the time. Sometimes this is easy – often it isn’t. It involves a lot of research, delving into resources such as Etymonline and historical theses.
Once you have written in a particular time period, of course, you get a feel for the language. You can just add a couple or words or phrases to your characters’ lexicon and the reader is transported to their time and place. Well, if you’ve done it well, that is.
There is always a danger of putting in TOO much flavor. Have you ever had a dish that was so heavily spiced that all you tasted was the seasoning, and not the food itself? Some writing ends up like that. Where you have to sound out the words on the page to make any sense of what was being said. I’ve seen some too-accurate Glasgow accents written this way. Or Cockney. Or deep south American. Just remember – less is more! And please don’t use phrases like “Avast ye, knavish varlet!”
Swearing is an area that is particularly difficult. A modern person swears differently than someone in the 18th century, 16th century, or the 5th century would. In the past, most swearing was religious in nature – ‘Zounds’, used liberally by Shakespeare, was short for ‘God’s Wounds’. Now, in a society less dominated by religion, we use words more related to physical body functions!
These are little things that must be kept in mind as you are writing your manuscript. Little but important. A glaring anachronism can push a reader right out of the story, and their suspense of disbelief shattered. Often small discrepancies can be forgiven (like rose madder being used to dye cloth in the 12th century when it didn’t become popular until the 13th). These are details only a historian or pedant will care about. Others, not so much (like horned helmets on Vikings). Don’t make your 12th century character a Baptist.