This greeting is pretty much the only Arabic I know, and it's important to me.
I use it all the time, typically with shopkeepers. Where once corner-shop proprietors were primarily Hindi-speaking people of Indian extraction in New Zealand, these days shopkeepers are equally likely to be Middle-Eastern, with Arabic as their first language.
The statutory response to this greeting in Arabic is "alaykum salam." Throughout my life people have taken pleasure from saying this to me, the unusual Caucasian extending the simple cordiality of greeting in the language, religious and cultural manner they are accustomed to amongst their own.
But there's more to it. Beyond simple courtesy to another person I do this because I owe a debt.
I'm a Westerner and for over a thousand years my ancestors committed the unforgivable crime of burning books: Hippocrates, Plato, Aristotle, Herodotus: any and every book which didn't align with the prevailing orthodoxy.
Meanwhile the ancestors of the Muslim minimum-wage shopkeepers I encounter in my daily life patiently preserved Western knowledge and culture in great libraries, and returned them to us when we crawled far enough out of the Dark Ages
mud to be ready to receive our own knowledge again during the Enlightenment
It saddens me that we have forgotten -or chosen to forget- the contribution of Islam to civilisation in general, and Western civilisation in particular. The single, solitary reason we Westerners have a cultural tradition at all is because Islam preserved it for us for a millennium whilst we were busy trying to work out how to eat rocks.
-SRA. Auckland, 24/ix/2020.