Interesting! There's a market for this kind of audio data entry? What was the total cost for that many hours? The English data was entirely volunteer driven, correct? Maybe it's worth funding the English corpus for the additional hours needed to reach the sweet spot?

Data cost plunges these days with self-supervised and semi-supervised learning. You don't need annotated and clean data anymore, there is abundance of it. Projects like Voxpopuli or Gigaspeech with 400 thousand hours (100 times more than Mozilla's) of data easily available.