How would you explain left and right?

Because you can.

I wouldn't get hung up on the specifics of the challenge. Decide for yourself what the minimum amount of knowledge, tools, history, whatever you & they would need to have for you to be able to explain left and right.

You can choose to answer the question assuming you are on Earth, know where you are and can see the sunset, but that wouldn't be very impressive because anyone could do that.

Instead, strip away as much as you can and figure out if you can do it. So, for example, if you figure out a way to do it where all you need is to be able to communicate in English, you should then ask yourself "what if we can't communicate in English?" and push yourself to devise a new solution.