Translation:The man who lives in Italy is a Canadian professor.
This distinction between restrictive and non-restrictive relative clauses is kind of superfluous in my opinion. I'm no English native and although I learned English for more than ten years at school, I never came across this one. I only became aware of it some years ago, and I really don't understand why it's so important in English.
It's certainly not a necessary feature for a language (most languages do fine without it, including my native language and apparently Finnish), I just personally find it nice to be able to distinguish between the two every now and then. :p
I wonder where it comes from though - as it's a punctuation rule in English, it's likely that it's a relatively new "invention" by the grammarians. (I think some language uses word order to mark the distinction with adjectives, but I'm not sure.)
It's possible but less common than "the man who lives" and it sounds a bit less formal (and possibly even slightly derogatory to some people).
Restrictive relative clauses:
people: who (that - less common, often proscribed)
things: that/which ("which" may be proscribed in some dialects)
Non-restrictive relative clauses (with a comma!):
people: , who ("that" is not possible)
things: , which ("that" is not possible)
Okay, so there is no "Canadian language" strictly speaking... Let's take French as an example then: I suppose "ranskalainen professori" means a professor of French nationality/origin. So how would one say "a professor that teaches the subject of French"? "ranskan professori"? "ranskan kielen professori"?