I hope that rate control would be dynamic, user adjustable during listening in private. Concurrent source sharing is problematic.
Internationalization needs mention.
In different windows, through different audio channels. The average text lengths for different languages differ, both because of different language issues of wordiness and redundancy, and the different translator's skills. With audio description, different scene and environment choices would undoubtedly be made for independent translations. Speech rate needn't be proportional to fundamental voice frequency. If word boundaries and punctuation were known, those allow ready places for changing pause duration.