A: Did you eat paella when you went to Spain?
B: Yes of course, when in Rome...
A: Why are you drinking wine, you hate wine!
B: Well we're on holiday in Paris and when in Rome...
A: Do you want to go surfing when you go on holiday to Sydney?
B: We definitely should, when in Rome...
What does "when in Rome" mean?
We say "when in Rome" when we are in a different place and we want to do the same things as the people in that place.
The full idiom is "when in Rome, do as the Romans do" but we usually just say "when in Rome"
We often say it on holiday when we try new food that is local to the country E.G Japan and sushi, England and fish and chips.
You could also say it if you start work in a new department and your new co-workers do things differently