Although it sometimes translates into tomboy, I don't think it's the same thing because tomboy also covers young girls who are particularly sporty and are boyish in outlook. Boyat is mostly about teenage girls and young women and is more specific a term.
Britexpat I found this article helpful when I learned the term boyat.
http://www.economist.com/node/15403091?story_id=15403091
Although it sometimes translates into tomboy, I don't think it's the same thing because tomboy also covers young girls who are particularly sporty and are boyish in outlook. Boyat is mostly about teenage girls and young women and is more specific a term.