Everyone present heard the apostles message in their own tongue. Why was this important when everyone spoke Greek? They all spoke a common language but when God wanted to speak to them he used their own tongue. As any bilingual person will tell you there are concepts that can’t be translated at merely a language level. It sounds like people were hearing things taught in a way they could understand which would include language and concepts. God seems to have a value for this diversity. Even the picture of heaven in Revelation is every tribe and tongue and nation. Why isn’t there one language even in heaven? Could it be that the Divine is to big to be expressed in the confines of a single language?