"It is a popular team in Ukraine."
Translation:Це популярна команда в Україні.
Very, very good question.
On one hand, it's not a strict rule. E.g. "Я у школі" and "Я в школі" are both OK (unless you are writing a homework on у/в interchange and lose points :D).
On the other hand, команда у Україні just doesn't work. Just. Ugh. Just even try to pronounce it, it sounds retarded X) Like, no matter how you ask I wouldn't add that xD
Unfortunately it's all messed up like that. A lot goes by the feel and not by the book, which is usually good, but not when you're trying to learn a language :)