Translation:They are eating salmon, potatoes and sauce.
Speaking from the UK, either plural or singular sounds natural to me. To say "potatoes" makes me think very specifically of boiled potatoes, which I don't think English-speaking people eat that often. The singular on the other hand covers many more forms when I don't know the context, like mashed potato, or baked potato, or something fancier like Potato Dauphinoise.
I think the closest counterpart to lox might be gravlax. Anyway it isn't a good idea to replace a general word such as lax, which covers salmon in general, with a very specific word such as lox, when translating here. If you find a restaurant in Sweden that actually serves lox, I'm sure they will call it lox here too.
I think in most English-speaking regions a "sauce" is a very general term for something liquid (or semi-liquid) to accompany other food, but "gravy" means a particular kind of sauce.
In the UK gravy was originally made by thickening the juices left from cooking meat, but often it comes from powder now. It would absolutely not be served with fish. I think "gravy" might cover a wider range of sauces in parts of the US, but would still be more specific than "sauce".