Món cá=fish dish (i.e. food)
Fish 'is' when talking about food.
This English sentence is a bit fishy
If it is fiah to eat then definitely fish IS ... I really don't expect the authors of the course to be perfect in english but where are the editors? This is the sloppiest duolingo course I've seen
This is a bad translation: It sounds like people like to hang out with fish as their friends. What the Vietnamese sentence says is that people like to eat dishes made out of dead fish.
The translation would be perfectly acceptable and understood in Australia. Similarly, when talking about food, we might say "Lamb is very popular in Australia".