We've just added support for internationalized URLs, showing unicode chars in domain names, path and query. Like in browsers, this is for display only. underneath the URL is stored normalized in ASCII format to remain valid and it is this one that is sent to your server.
You can paste an IRL when creating a new check, updown will accept it and convert it to the appropriate format automatically (this is also true for ASCII characters which are not allowed in URLs).
It's possible there are still some places where the ASCII version will be shown instead of the unicode one. As always, let us know if you have any feedback about this new feature.