This brings us to the common expression “Pardon my French” or “Excuse my French.” Today, it's a warning that you are going to swear or use obscene language, and it has been used to mean this since about 1895.12 Jul 2018
Why do we say Excuse my French when we curse?
The phrase was originally used in England when someone used a French word when speaking to a person who may not have understood French. Due to the history of conflict between France and England, 'pardon my French' came to be a dig against the French.