Welcome to the Forum!
Sadly I don't know the first thing about opening hostels in Japan, but if you already know the city where you would like to open that will help you in the process. The local Chamber of Commerce (assuming some kind of equivalent exists in Japan) may be able to help with some of the questions that you posted. A phone call or an email might provide some answers, even while you're still in France. Another source of basic information would be a hostel in another city, with whom you could work in the future and for whom you would not appear as a competitor. Even if they can't explain every possible option in detail, at least they can share their own experiences and tell you about their own choices and challenges.
I wish I had more concrete information to offer you. Perhaps as you find the answers to your questions you can post them here.
All the best in your new adventure!