I reported it too. I wrote "Why don't you speak English"? but got marked wrong.
Medabrim (מדברים) suggests there's more than one person involved, as it's a plural form of "speak". We don't have a pronoun here, then this could mean "we" or "they". ;-)
I'm with you on your reasoning. I wish a native speaker could chime in here. I think they chose "we" because in the simplest context of a conversation between 2 people not involving/referring to anybody else, it would have to be we.
This is the reason why you think he's talking too fast: https://www.psychologytoday.com/us/blog/life-bilingual/201302/why-are-they-talking-so-fast
Why can't onw use "you" in this instance? Got marked wrong for translating the sentence thus: "why don't you speak English?"
It's kind of a contextual grammar thing. When a pronoun isn't used, and the sentence is using plural verbs, the implied meaning is something like, "Why does one not speak English?" Or in this case, "Why don't we speak English?" It's kind of a rhetorical phrase, much like how we use the word "you" in English. For example, "You don't do it that way," doesn't necessarily mean you are telling someone not to do it that way, rather it means that it's not the standard way of doing it.
I hope that's helpful