Yes, indeed. I think that this question might apply in some ways to both fantasy and science fictional scenarios. After all, you're dealing with a completely foreign environment in which all kinds of things are different and unexpected. How the heck do you write "what you know" then?
Well, one answer is that you can always learn things through research in the scientific or folkloric arenas required by the fantastical setting of the story. That is a pretty straightforward answer, and always worth pursuing. Indeed, I recently read an article that suggests we keep things as "real" as possible in science fiction and fantasy worldbuilding. It's an excellent point.
A more fundamental answer, though, would be that many "things you know" are hidden just underneath all the foreignness. When I read a story, the thing that strikes me most strongly is usually not the trappings of the environment, but the nature of the human experience that I'm sharing. That "human experience" is something I know. Even my aliens have human-like experience and psychology, and emotional states and reactions that resonate as familiar. Otherwise I'm not convinced anyone would want to read stories about them!
Here are a few examples of "things I know" that I have put into stories just by adding an alien twist:
- I know what it is like to speak a language and not have a native speaker recognize me as a legitimate speaker. I put David Linden in precisely this situation in "Let the Word Take Me."
- I know what it is like to be treated unfairly, like a second-class citizen, and not have any reasonable recourse. I put Rulii in this situation in "Cold Words."
- I know what it is like to have a superior not understand the worth of my contribution to a project. I put Lynn Gable in this situation in "At Cross Purposes."
- I have personally witnessed the in-between culture that can form between foreign visitors to a country and natives of that country - a context in which actual cultural engagement is not welcome. I am putting Adrian Preston in that situation in my story-in-progress called "The Liars."
It's something to think about.