I was using "worldwide" with some amount of journalistic license. I would not expect people in Africa or S. America to follow the USA in most subjects, let alone our form of "futbol". I have traveled extensively in Western Europe, the UK, Eire, SE Asia and Japan and it is somewhat shocking (to me) to see NFL team logos worn by natives and some NFL games (or accounts) televised on local TV. I had no idea the NFL had that sort of following.
Clothing with American sports teams' names and logos is popular in many countries, especially among the young, but the attachment is only to the fashion, not the sport. The NFL has built something of a following in England, and the games played there every season sell out, but American football is nowhere close to challenging association football there or on the Continent for sheer numbers of fans or deep-rooted loyalty. You may see a young couple on the streets of Munich wearing matching Patriots Brady/Gronkowski replica jerseys, but ask them to name the team they care about most and chances are nearly 100 percent that they'll say "Bayern" and not "New England."