"The coach is full of people."
Translation:Autocarul este plin de oameni.
It's all about the context. In English, "people" can mean several things: it could either be the plural of person, OR it could be a collective noun describing a group which share a common nation (or culture, language, tradition etc.). The first one can be translated to "oameni", while the second one can be translated to "popor". In the Duolingo sentence above, only the first meaning of "people" makes sense, so it's translated to "oameni".