For example, if there are two samurai, is it "samurais", or "samurai"? I'd love to know the rule for that. Thanks.
Samurai is the plural of samurai. Obviously there isn't one single rule for all "foreign" words, though.
You should use whatever rule is correct for that language, if it will not confuse the reader anyway. Japanese, as well as some other Asian languages don't have a plural in the same way English does. With Japanese, there is no difference between the singular and plural in any way that you would be able to use when writing in English.
It depends on the language (unless you really want to write something that no-one can understand, like someone I know did when I tried to teach them Gaelic ), but in Japanese there's no plural, as far as I know.
I would use the plural of the language, unless the word is already well known in English. I would write kocav as kocavim and ma'or as ma'orot, which is Hebrew for star, stars and luminary, luminaries respectively. With Japanese, perhaps add an "s" to words that are less known.
It's really about what wouldn't confuse the reader. Those Hebrew words are not widely known, like kippa/kippot (the little cap men wear), but as long as the context make it clear that you're talking about more than one of that object/person, I'm not sure if it matters how well known the word is.
That's true. "That kocav is beautiful." "All the kocavim are beautiful." I like the Myth Busters' quote, BTW.
I agree with this. I would also consider researching foreign words yourself so you can also understand what the plural is in the first place
