LordCrash it's not illegal, but you would need the permission of the copyright owner, and 1) they are unlikely to give permission because that would be seen as ripping of their Japanese user base and 2) if they did give permission, there would be no point since they could just give you the original files.
But creating such a tool and releasing it is perfectly legal, and it would be legal to run that tool on your own legally acquired videos.
LordCrash I highly doubt that tbh.
It's true.