I'm from Bulgaria and we use Cyrillic alphabet.
For good presenting and SEO I use Cyrillic slug for my post and pages, but there is some problem with cyrillic page's slug.
The bug come when I change the Permalink Structure to "/%category%/%postname%/".
For me that is perfect structure, it work like that:
But when I put Cyrillic in my pages's slug it go to 404 error page in my blog. There is no sense how or where I use it in my pages (page's only!).