One of my favourite things about fiction is the ability to create my worlds, my way. The Egyptian mystery series I am writing does require historical accuracy on many levels, but I still have the ability to create characters who could have existed. Adding a fictional character into a real event in time just deepens the layers of the storyline.
I have outlined another series of fictional books, set in and around the lost civilisation of Atlantis. With that series, I get to actually create their world. It sounds easier than it is. I have named the days of the week, sketched out their religious beliefs and their deities, created unique names which will identify them as Atlanteans, and have even made up a language for them.
But, I do have to be careful how many "foreign" words I introduce in either one of my fictional book series. A few well-placed words add flavour and ambiance to the work, too many make it cumbersome to read. Some words just seem to sound better in ancient Egyptian or Atlantean; they don't seem to translate as well into modern English.
So, if you are writing historical fiction, take the time to really learn your time period. Speech patterns, idioms, even actual words, need to be true for historical accuracy. And, when working in that time period, you can make up your own idioms; who knows if they were used - well, you do, because you are the author and you have created this wonderful world.