Where exactly does it transform? What did you try and didn't work?
As far as I can see, the URI class has no issue with any utf-8. As you say, Fuel is used a lot in Japan, if this would have been a framework issue, it would have been reported years ago.
So for the moment I think it's either your code or your development environment. For example, are you perhaps using Windows, and trying to use utf-8 in filenames?