Very understandable. There are so many interesting expressions in every version of English even within one country. I find them delightful, though at times confusing. True story: "An Am. invites a Brit. for dinner. Since the visitor has been traveling the host asks if he'd like to wash up before dinner. The Brit. guest replies: "Well, I usually do that after dinner." ;-))
While what you have written would be understandable and is used in casual conversations in a language learning course based on translating sentences it's vital to translate fully. So, "are" is required. Yes, it is grammatically incorrect in English. Oh, and "rimangono" is a verb so it should be reflected in the translation.
There are a lot of translation websites out there, although I don't fully go with what they say. Oftentimes, I will look up the word in my "Barron's Italian-English Dictionary" and see if it makes sense. 'Rimanere' from which 'rimangono' comes, means many things: to stay; to remain; to be offended/confused; to be amazed. My former Italian teacher told me there are something like 120,000 English words and approximately 50,000 Italian words. I guess that's why there are so many meanings for one word. However, I do think it's confusing and I also think this Duolingo should list all the meanings, so we don't get statements incorrect!
As an EFL teacher I'd have to disagree with your suggestion to include all meanings. There would be nothing but pages of synonyms and lots of time spent on perusing them when we know we won't remember but a few and those being the ones directly relevant to the sentence. Duo does well to stick to three meanings and it's usually the first that fits the situation at hand. For other words we have a host of dictionaries. Try this multi- translation site:
And this for hints and the all important Guidelines: