Interestingly enough for a Swiss, he uses a form of Parisian street slang (which has now spread all over France ... and beyond) called "Verlan,"  which means "l'envers," or "inverse" in English, and its main rule is the inversion of syllables.
Since it is an oral slang, it uses the spoken version of words and not the written one (as you may know, in French one does not pronounce all the letters written in a word).
Also, because some of the words are difficult or ugly to pronounce when inversed, over time they got tweaked into close-sounding words - this introduces another degree of difficulty as you just have to know those and the "inverse" rule does not really apply. Best example for this is "flics," which means "cops," which became "keufs" (shorter version of 'keu-fli,' which would be the literal Verlan).
Now as to what this footballer said, it is not exactly what I have seen mentioned in some of the press, here is the original tweet:
"Je fonsde out les coreen allez sout vous lebru. Ahahahhahahaah deban zotre"
Which in 'normal' French, and correcting for the numerous spelling mistakes, would be:
"Je défonce tous les Coréens, allez tous vous brûler. Ahahahhahahaah bande de trisos."
And my translation would be - sorry for the language:
"I fuck up all the Koreans, go burn yourselves. Ahahahhahahaah bunch of retards."
"Go burn yourselves" could be a reference to the fact that when you are completely desperate and with no hope ever (ie because the Swiss football team is sooooo good...), your last resort is self-immolation, also an insidiously racist reference to Arabs (thru the Arab Spring events).
"Trisos" is short for 'trisomic,' probably one of the worst and most denigrating insult to use.
So in a short tweet, he managed to offend Asians, Arabs, and people with disabilities, what a brilliant effort.
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