More YA books feature white main characters than characters of color. You’re right. But that means there are a lot of editors (and agents) out there who are looking for books with multi-cultural characters. They just have to be good (well written and high concept).
And urban language could limit the audience if it will make the book harder to relate or understand. (Language, accents, or dialects can do this). But I don’t think settings should be a problem. And if you weave in the language well so that it’s authentic without being overbearing (sometimes in writing it’s a balance you have to strike).