In the time of Cyrus and Darius, when the government was still Susa and Elam, the language was Elamite. This language is extinct and it was it was spoken between 2800–550 BC. This language was mostly written on stones. This language was not a standard language. In 484 BC, when the conquest of Mesopotamia happened  a new language was used, the Aramaic language. Aramaic language have an antiquity over 3000 years. This language was used for speaking and written communication between the empires and the people. Some books like Daniel, Ezra and Talmud are written in Aramaic language. The new Aramaic language is spoken by Assyrian, but it counts as an extinct language. This language is form the Afro-Asian family. Old Persian was a great language, you can see them on projects and arts etc. But old Persian language was so incomplete and so far from perfect. So as the time past, the beautiful Persian language developed and turned to an incredible language that we call farsi and use it in present time.