Right now it seems to be that a slug is unique per page.
It would be really important for multilingual websites to have different slugs (aka friendly URLs) for different translations.
Example (for a hotel specific website):
System page type room list:
http://server/rooms (if english page is requested)
http://server/zimmer (if german page is requested)
those 2 URLs should lead to the same page ("zimmer" means "rooms" in german)
2nd example: specific room page
http://server/rooms/double-room (if english page is requested)
http://server/zimmer/doppelzimmer (if german page is requested)
those 2 URLs should lead to the same page ("doppelzimmer" means "double-room" in german)
It may also be useful to support language codes in URLs (alternative would be to use the Accept-Language http header for the examples above).
In some cases the language might not be determined by the slug (or the slug is not translated)
The "language code in URL" may be easily solved by defining routes for that (and then set the Accept-language HTTP header based on the detected language).