Why, upon saying a naughty word, will someone then say “Pardon my French?” Is French a vulgar language?
I always thought that it was an opposite statement since French is an elegant language.
Kind of like someone farting and saying, " Excuse my perfume."
No. The idea is that rather than having said a rude English word, he accidentally lapsed into speaking a (presumably polite) French word, which may have coincidentally sounded like a rude English word. Wink wink. The premise is that the listener doesn’t know French and thus can’t be sure that the excuse isn’t true. Of course in real life we know it was a rude English word, so it’s more of a stylized way to say “Oops! Sorry I cussed!”
Because the French were considered the epitome of everything lascivious and risque. If you said something naughty, you excused it by saying it was French. Also, naughty things were often referred to as French, or the French term was used. French kiss, French letter, doing it Frenching, soixante-neuf.
Could it have anything to do with the French phoque /fɔk/ (seal)?
(Probably not, but it’s a nice coincidence)
In his diaries, Pepys resorted to French when he described anything risqué. I imagine that this was a common ploy for educated people at that time and the practice has morphed into the present day euphemism.
Also, and I think Cecil has pointed this out, France and the Netherlands were England’s big rivals at one time, so a lot of English idioms containing “French” and “Dutch” are primarily disparaging, and may be only vaguely connected to actual characteristics of the people. Consider French leave, French letter, French kiss, Dutch courage, Dutch treat, in Dutch, double Dutch.
Plus French was the dominant cultural language of the area. France was (allegedly) more cultured; educated people spoke French, important people went to France quite often, it was the nearest neighbour, etc. So if you lapsed into French, it meant you were classy… as opposed to sounding like a longshoreman.
PS. search YouTube for “Family Guy French lesson”
“And remember kids, you can use it at the dinner table, because it’s French and French is classy.”
These are the correct answer, the others talking about French being classier are incorrect.
Although, to be fair about it, French is considerably earthier than English. You might want to look at a couple of great books called Merde! and Merde Encore! for examples.
I am really fond of two French idioms: a French woman getting her period would say (curses on trying to make this keyboard type in French) “Les Anglais sont arrives.” Loosely translated “the redcoats have landed.”
Also the French phrase for a woman with a large bosom: “Elle a du monde au balcon.” (She’s got a crowd in the balcony.")
These are pretty clean. But we English speakers would be a lot more cautious about referring to a real mess as “une tas de merde” (a pile of you-know-what) in relatively polite company. Not so with the French.
I note that ordinary commercial transactions (buying a croissant and a cup of coffee, for example) are considerably more formal than in America. So it balances out…
This reminds me of a post I made on another forum and which, since the OP’s question is answered, I reproduce here:

But we English speakers would be a lot more cautious about referring to a real mess as “une tas de merde” (a pile of you-know-what) in relatively polite company. Not so with the French.
Or indeed the near relation for anything untidy or generally disorganised - “Mais c’est un bordel!” / “Il est bordélique, ce type-là!”