"اَلْسَّلامُ عَلَيْكُم! وَعَلَيْكُمُ ٱلْسَّلام!"
Translation:Peace be upon you! And upon you be peace!
People might think it's a pain to have to translate the whole phrase, especially with the specific grammar of the second phrase/response, but it's actually pretty important to know what the phrase actually means. Its meaning carries a lot of weight. It's a phrase used as a greeting by Muslims; it's not "hello", as much as used in place of hello. "Hello" in Arabic is "marhaba" as mentioned in this lesson as well. As a Middle Eastern Christian, I actually never use this phrase when speaking in Arabic. I exclusively use marhaba, or foreign words like "hello" or "bonjour".
So yeah, long story short, it's a pain, but it's important! Don't get lazy :)
I think the literal translation should be thought about and remembered, it is said for a reason and a lot of people mean it when they say اَلْسَّلامُ عَلَيْكُم or وَعَلَيْكُمُ ٱلْسَّلام . Otherwise people would just use the less formal "Salam" literally meaning "peace"(which they do when talking to close friends). But at the same time, Duo should also accept answers with hello and fix the translation of the 2nd sentence to something that sounds more natural.