It's probably not only the audio files, but also thousands of names and other wording and their links that change with language. Actually, I doubt that much of the audio files would change; whatever the base language, one would expect that when playing as the Romans their response to your commands would be in Latin, for example. As to the commentaries when a new tech is discovered, I don't know if those were recorded in languages other than English - but doubtless somebody knows.