it would generally carry the same connotation, but the literal translation using "nel" would mean "I am in the restaurant". it's a slightly different meaning, since "at the restaurant" could mean at the door (outside), in the parking lot, etc.
Why does duolingo say "AL ristorante" then? Is that the literal translation for "at the"?
I'm guessing here, in fairness, but much as in contracts to absorb il into nel, lo into nello, so I'd assume that a would contract il to al, and lo to allo.
Correct me if I'm wrong.
could that be "Io sono nel ristorante"?
couldn't it be "io sto al ristorante" also?