That is a laughable translation. No native speaker of English in the US or UK talks like that.
Lol. I enjoyed Soglio's comment but for a language learner, Jones_Rick is correct.
No matter how many times I listened I could not get parli just something that sounded like far le
You can say "What are you talking about?" (You need "you" for the subject.)
"What do you mean?" is a slightly different question.
[Native US speaker]
"Stop the clutter" is a reasonable warning but your translation “Of what do you speak” is not only true and correct translation and is not the best one. “What about are you speaking?” or “what about do you speak?”, or "What are you talking about?" were not accepted :-) Duolingo-team, please arrange..
"of what do you speak" is perfectly correct English, although it sounds a little formal. "What are you talking about" is fine, but your other two suggestions sound wrong to me. "About what are you speaking" would be fine, although again a little formal, but I don't think you can switch the "about" and the "what." (Though this is my American ear - maybe Brits, Australians, or Canadians would have a different view.)
Native English speaker from Australia and I definitely agree those other two sentences don't sound correct to me at all