[eluser]Phil Sturgeon[/eluser]
This is of course difficult as the URL segments map to PHP classes and functions, which are restricted to a-z & underscores.
You can enable UTF-8 characters by entering them into the allowed_chars_list in config.php then set up routes for your Greek URL segments, or by patching the router to convert UTF-8 characters to their nearest a-z characters perhaps?
Either way, no easy task.