My German friend tells me that no one in Germany ever says "das Fischsandwich". The correct term is "das Fischbrötchen".
Hi, I'm German. There is nothing wrong with 'Fischsandwich'. (Try a google search) BTW: 'Brötchen' and 'Sandwich' are not equivalent in German. We use 'Brötchen' for bread rolls, so the word does not include 'two square-shaped slices of bread' for which we use 'Sandwich'.
I'm in NRW and people do and will say Fischsandwich, it's just that nobody eats them. I haven't ever seen one single German eating an actual sandwich. They mostly just eat rolls and pastries. Which is something to say about the lesson. The lesson itself should say "Fischbrötchen" because you're eleventy billion percent more likely to be ordering ein Fischbrötchen and not ein Fischsandwich.
I'm aware that they're not equivalent. Whereabouts in Germany are you from? It might be a regional thing.
I'm from the south-western part of Germany. But I don't think there are many regional variations concerning the use of 'Sandwich'. However, there are strong regional variations with respect to the words we use for bread rolls. Perhaps your friend had a special context (or a special type of sandwich) in mind.
@rymd: wataya is right; it's not a regional thing. Fischsandwich is fish sandwich, while Fischbrötchen means Fish roll; different things..