“I do not speak French” versus “I do not know how to speak French”. Both are correct, though only the latter clarifies not speaking the language because they do not understand it, rather than purely out of spite. So in this specific case, the former could be used as a subtle FU.
“Je ne parle pas français” There you go, everything you need.
IIRC if you cannot do it because you never learned it it’s “Je ne sais pas parler français”
“I do not speak French” versus “I do not know how to speak French”. Both are correct, though only the latter clarifies not speaking the language because they do not understand it, rather than purely out of spite. So in this specific case, the former could be used as a subtle FU.