I have no idea about the training industry in Japan.
I suspect most folks either learn to fly in the US or Australia.
A degree is not required to fly an aircraft. Note, the majors in the US usually expect you to have a degree.
If you're after a sponsorship, then many applicants already have one.
So, if you're not planning on going for a sponsorship, then you don't need one.
I would investigate local sponsorships if you can. See what they want and if there are any going on.
I only know the normal expat path. Most learn in their respective countries, Europe, Australia, New Zealand and the US to name a few. They work their way up through the GA or instructing scene, charter and then regional airlines. Most asian airlines expect you to have quite a few hours.
The alternative is the military path.