Hmmm, I thought fish was one of those English words that is both singular and plural
Yes, you're right. The word "fishes" is typically only for referring to five species of fish, not five individual animals of the same species.
The reason you can't right "The fish are Norwegian" is that fisken in Swedish is in the singular.
Why not "The fish are Norwegian." ?? In English fish is both singular and plural.
Någon ring den Svenske kocken omedelbart! Det är den sällsynta, smakliga morska norska norrsken torsken!
("Morsk" means something like 'boldly unafraid and self-confident, often bordering on aggressive'. Something like that. I checked. And it's pronounced funny. Almost like there's no 'r' in it.)