At first glance, superheroes might seem like a strictly American thing. That's technically not correct because over the years the two big comic companies have created a number of International stars.
DC's created a United Nations-like group called Global Guardians. It included Green Flame/Fire (Brazil), Ice Maiden/Ice (a Scandanavian country), Tasmanian Devil (Austrailia) and Jack O'Lantern (Ireland) plus several more.
As I wrote, they aren't the only example either. Just in the last 20 or 30 years, they've really ballooned.
In the mid 1990s, Alpha Flight went on a "world tour" where they met up with several international heroes.
Thanks to the Cold War, there are tons of Soviet supers.
There are bunches of Japanese heroes, mostly thanks to its "hipness" in the 1980s.
Marvel's Contest of Champions introduced a number of international characters as well. A good place to find later appearances of "Contest of the Champions" heroes are in Marvel Two-in-One and Marvel Team-Up. They were always concocting ways for Thing and Spider-Man to meet these folks.
Technically, Scarlet Witch, Quicksilver, Iron Fist (I think he's English) and Nightcrawler are foreigners too.
Both Marvel and DC mostly seem to prefer to make up heroes and villains who reside in fake countries — Dr. Doom, Sonar, Aquaman, Wonder Woman, Geoforce, the Savage Land heroes, the villains of DC's Bialya are all non-U.S. citizens. You could even include characters who come from secret civilizations such as Wundagore Mountain as U.N. heroes.
Include the alien element, such as Martian Manhunter, Starfire and even Superman, you technically have a lot of diversity in most teams.
Despite all this, many of these characters have been written as "American" for so long they really don't count as being a international character.
In the "Justice League" book, for example, a letter writer once asked why Fire doesn't speak with a Brazilian accent. The editor replied (jokingly) that the Letterer kept editing her dialogue.