This reflects a larger issue about project management: Do you want more people working a little on the project or a few top performers working at their absolute

This reflects a larger issue about project management: Do you want more people working a little on the project or a few top performers working at their absolute maximum productivity? If I was them I'd keep running the project in the way that makes the top 10 contributors as productive as possible, and it sounds like keeping the discussions in Japanese is the way to go. Think about if we were discussing Apple. Would you really suggest it would be worth making the top performers (including Steve Jobs) do their day to day work in Japanese if it meant more people could participate in the discussions? Or do you want to do everything you can to have the people who really know what they're doing working at their optimum?