You just use a (somewhat cleaned up) UTF-8 representation as a slug. UTF-8 URL components are universally supported (transparently encoded and decoded) in all modern browsers, and Google shows the real characters instead of the urlencoded version. You still end up with the urlencoded version when you hit Ctrl+C, but I expect that to be fixed in the near future as Unicode becomes even more widely used.

For example, the following link should work perfectly well in all modern browsers:

https://ja.wikipedia.org/wiki/メインページ