Where were we? After C like coffee there D like Diner!
There is not a Italian word to translate this term accurately. Because, as many other things described here, this is an entirely American idea. But the Italians will be somehow familiar with it, because they have seen diners in any TV-series they have watched.
For any Italian visiting the States, eating in a real Diner, will be one of those things-you-must-do
Especially if the place is a pretty vintage one.
They will surely mention at least one or two tv-series in which they saw a similar place
They will notice that servers wear a uniform or a t-shirt of the same color and the name tag “Cool! Just like in (tv-series/movie)”.
They will also claim that servers normally introduce themselves to customers (nasty comparison with Italian servers to follow).
They will praise free water and free refill. “How cool!” they will enthusiastically comment.
They will probably be a little be nervous when the waiter will bring you the bill, because they will interpret this as an invitation to leave. Reassure them.
But also remind them to leave a tip, or they will not do it.
Prepare yourself because they will ask you advice about the most typically American food to and possible complain about the bread.
I bet your Italian friends will order a Sandae for dessert
and they will be absolutely ecstatic when they will learn that diners are open 24/24.