When we swear in English, why do we say Pardon my French?

It is said that the French language is used because of the association of the French people with vulgarity. An innocuous theory is that when the English were looking around for a foreign language to put into the phrase "pardon my ...", the closest one and obvious choice was neighbouring French. Additionally, French was for a long time the most-spoken foreign language in the United Kingdom. Some believe the expression may have come from 1950s intellectuals who were well-versed in French. In the fifties, cursing was considered to be a major taboo, so the speaker would actually curse in French to be more polite. In these instances, "Pardon my French" would refer to actual French.