Finding Nemo is being released at staggered times throughout the world. It wasn't released in the U.K. until October 10th, it will open across continental Europe over the next two weeks, and won't open in Japan (the largest non-Enghlish speaking market for U.S. movies) until December 6.
Openings are delayed like this to give distributors more time to dub films, make prints, prepare marketing and so forth. There's also some seasonal issues that vary from country to country in terms of when the big office hits. I think the U.S. is the only place where summer is a huge movie going period - other countries have different markets.
It used to that films had even longer release times in international markets (some times measured in years). But with the growth of DVD and truely international media companies that period is shrinkign all the time. The last Matrix was released at the same time (literally down to the hour) across the globe.
In terms of the actual movies themselves, action really does sell better internationally than comedies. Explosions don't need a translation; too many jokes are based on culture, history or countries personality. It's extremely difficult to make a British comedy work in America. Now imagine how hard it's going to be to get California surfer dude turtles to play in the Ukraine.