Exactly. From wikipedia:
Originally Posted by Yfelsung
Around 500 BCE, following the Achaemenid conquest of Mesopotamia under Darius I, Aramaic (as had been used in that region) was adopted by the conquerors as the "vehicle for written communication between the different regions of the vast empire with its different peoples and languages. The use of a single official language, which modern scholarship has dubbed Official Aramaic or Imperial Aramaic, can be assumed to have greatly contributed to the astonishing success of the Achaemenids in holding their far-flung empire together for as long as they did".
Given the above, and given that we are seeing a "historical" representation of Darius as he would interact with other cultures, why would we expect to see a language other than Aramaic?