Start with genre. Are you writing action-drama? Fantasy? Economic philosophy? That's your PRIMARY audience, I think. People who read magical fantasy can handle Harry Potter even if they're in their thirties, despite that it's a kid's book.
After that, I think you can afford to be pretty general. You don't have to write a best-seller on the first try (which is good, because you won't). Pick a group that interests you and start targeting them with your writing. You'll get better at it over time. If you want to write to young adults just do it, and then show it to some young adults and see how you did.
Now you've got that trilingual thing to worry about, which I only sort of identify with because I'm nowhere near fluent in any of my.... three? Four. Four other languages -- I can speak some but I could never hope to write a damn book in Japanese. So my inclination would be write it in English, and later if you feel so inclined, translate it -- of course that's not necessarily perfect. If I was going to write a Meiji romance, I'd take the time to learn the native better and write it in Japanese. But that would be for a specifically cultural piece.... I think you'll find that the professional writing world is competitive enough, you'll want to take every advantage you can get. I'm sorta assuming you're best at English.
The most important part is always just to write. You can market on the second draft -- get the first draft down on paper before you start stressing out about who's gonna buy it.