If writing dialogue set right now in the real world is difficult, writing historical dialogue is even harder. Surprising as it sounds, you don’t want absolute accuracy. The deeper into the past you go, the less likely it is that anyone living today actually knows how anyone spoke and the chances are, we wouldn’t understand them anyway.
What you do want is readable dialogue that carries information, advances the plot, indicates relationships, conveys character, mood AND is authentic. My fourth novel, Sugar Island, is set around 1860 on an island off the coast of Savannah, Georgia, where my English protagonist, Emily Harris, has been forced to live by her American husband on a slave plantation. Emily is based on the real life actress, Fanny Kemble, who wrote a diary of her experience on St Simons Island.