Brooks got away with the constant use of the word “Nigger” because Richard Pryor was credited as a co-writer of the script.
Why do they call it French Kissing if us Canadians do it best 
Duckster, I like your answer…cute. mmmmmm
The English language has several slurs based on nationalities. You welsh on a bet and drink your Dutch courage. In the Napoleonic Royal Navy, Irish pennants were rigging that were repaired by being tied together rather than spliced.
The fact that the French are often perceived by English-speaking peoples as “racy” is no doubt a factor.
Another is the fact that people generally have a habit of ascribing things which are part of their own national experience, and which they they find to be embarrassing or controversial, to someone else. I’ve read that Russian Roulette is sometimes called Spanish Roulette in Russia. “Syphilis” was originally only a given name in Greece, while the Israelites, when speaking of venereal disease sometimes referred to “the botch of Egypt”. I expect hundreds of other examples can be cited.
The French they are a funny race,
They fight with their feet, and make babies with their face.
Or something like that.