in a normal situation you can say "Apa yang sedang kamu bicarakan?" or for formal way you can say "Apa yang sedang anda bicarakan?"

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what are you talking about? = Apa yang sedang kamu bicarakan?/ Apa yang sedang kamu omongkan?

you can say : a) informal situation : apa yg sedang kamu bicarakan? b) formal situation : apa yg sedang anda bicarakan? or Anda sedang membicarakan apa ? Nb : you can see the different between informal and formal is how to use "you" informal situation you can use "kamu" formal situation you should use "anda". Anda is more polite than kamu or kau or engkau. Hope this can help you to understand how to use "Anda" and "Kamu or kau or engkau" best regards

Apa yang sedang kamu katakan / omongkan/ bicarajab.