If you write it in the language you're comfortable with, you'll get the idea out and find the perfect words for it later. (I know how hard that is. Trust me. I have to scroll to the bottom of my document every time I open it to keep from going back and nitpicking over my own writing. I had to set up deadlines and then my boyfriend had to threaten to lick my eyeball to get me to keep to the deadlines and not keep revising - because, really, who wants their eyeball licked, am I right?)
Another alternative is to take a break and hit the library, or alternatively, Netflix. I'm writing a story set sort of in the same time era as you mentioned, and I find Charles Dickens to be very helpful in getting my diction in the right frame of reference. There are things that might be off here and there, but a good few read-throughs by people you trust will alert you to anything that sounds too false. Reading/watching an adaptation (most particularly reading) will immerse you in that time frame.
For instance, have you read Jonathan Strange and Mr Norrell by Susanna Clarke? Aside from being a gobsmackingly good read, Clarke writes it as thought it had been written during the Napoleonic Wars, so it gives that weight of historic accuracy to it and grounds the fantasy plot within real events and authentic language. Alternatively, Naomi Novik's His Majesty's Dragon series is far more modern in its language and plot structure, but the historic details (even when they deviate from true history) are so sharp and precise that it gives the same weight to the story, just at a different angle. Either book feels like an alternative slice of that era of history, but use different tools to accomplish it.
So it depends on what you're trying to achieve, really. Do you want it to feel like a book that could have been written back then, and later "found" by you? Or do you want it to be more cinematic in scope, modern in its faster paced plot and witty dialogue? There's lots of different ways to go about it, so you have a lot of wiggle room.
