I am, admittedly, not a big soccer fan. I actually find soccer a little boring and my watching of soccer is relegated to huge matches (Olympics, World Cup, etc).
That being said, I believe the World Cup should be just like the Olympics and you should be rooting for your country or adopted country.
Like me? Born in the good ole United States of America. Guess who my rooting interest lies with? That's right, big surprise, the USA.