What is the difference between the phrases אל מדינה ומדינה ככתבה, to each country according to its writing, and אל עם ועם כלשונו, to each nation according to its tongue, found in Esther 1:22 regarding the decree to kill rebellious wives? Both phrases seem to indicate that it was sent in each country's language.
R. Yosef Kara to this posuk (Nusach Alef) says that this decree was sent in the ("official") language of each country and then the designated translators in those countries would translate the content to the other languages of peoples living between them.
ומדבר כלשון עמו - המוליך את הכתב יודע לקרות את הכתב ולדבר ולתרגם את הכתב כלשון העם
Each state's script, and each people's language.
An empire could easily have multiple governing units that don't necessarily correspond to ethnic boundaries; it could also have multiple alphabets which are used for multiple languages. Consider for instance Nigeria, which contains many different peoples with many different languages, but the entire state almost exclusively uses the Latin alphabet in written form.