"Speak Greek" in Chinese In English, when someone says something incomprehensible, one can say they "spoke Greek". In Italian, the equivalente is "parlare Arabo" (lit. speak Arabic). What about Chinese? Is there a similar idiom?
说天书/听天书 would be proper one in my opinion. 

It means one talks about the book from the heaven and no one would actually understand it.

For example, 他讲的话， 我就像听天书一样。 我听物理课像听天书。 他在说天书。