He speaks a shit ton of languages (it's thought that when he lists the stats of the other experimental soldier group in Civil War, and he lists that they speak 30 languages, he was in fact just listing his own stats). Though his Mandarin is like. Prolly from the 1970s or something? So they may or may not be able to really communicate, given Cao Pi's time period XD

If it's close enough to be compatible to modern Chinese, though, it's probable that he is in fact speaking it at Cao Pi right now. He always tries to speak the same language as his conversational companions. I just don't know if it's close enough to be compatible.