jeap, this is a good question and we discussed it in this thread (see comment 4 from me for example).
What we found there was: Almost every challenge is global as well. That is the trick of an online platform - that people from all over the world can contribute or use that solution. So with a "Global" category - everyone would be confused - where to post the challenge - in the city or in the global category?
So the idea was to say, that the challenge should go to the city where the people that are willing to work on it, live. Because that people are the most important resource of that challenge. And in that case this would be your city, right?
There is also the possibility to use the "other" category.
That solution is not perfect. We still need a better idea. What do you think?