Sport vs. Sports
I can't speak for British English, but in American English, we never call sports "sport," we always refer to it as "sports." For any American using Duolingo, they will always get the answers translating "le sport" as "sport" wrong, at least on the first or second (or third) pass.
I don't agree. I'd say we can use singular and plural like most nouns, and also "sport" as a singular mass noun, depending on context. E.g. Rugby, tennis and golf are sports. Call of Duty is not a sport. I don't really play any sports and rarely watch sport on the telly.
I'm not just learning French, Spanish, Portuguese and Italian here on Duo. I'm also learning how much English spoken in the United States differs from the English spoken elsewhere. Heck, I've even discovered that there are minor differences between people who live on the East Coast of the United States vs the West Coast of the United States, for example. I've also noticed that the older the person, no matter where they're from, they're more likely to be a prescriptivist, and the younger the person, the more they're apt to be a descriptivist.