Hemingway set his stories in many different parts of the world, as pointed out by bullgatortail above. This use of setting lends his writing some international appeal  as does his interest in the battles and conflicts of World War I and the Spanish Civil War. 

The sense of creeping disillusionment that undergirds much of Hemingway's work probably also helps to create a general modernist appeal in his writing. 

In part because he is one of America's greatest writers, Ernest Hemingway's stature as a literary giant has reached around the globe. National boundaries do not contain great writing, and Hemingway's popularity in other parts of the world are a testimony to the quality of his prose. I think his own personal exploits helped to create a larger-than-life image that became attractive to many prospective readers; but more importantly, Hemingway's settings encompassed many locales around the world. His stories are set in many different places, from the mundane to the exotic: Middle America, Florida, Cuba, Spain and Africa are just a few of the places that both Hemingway and his own fictional characters inhabited. Combined with his knack for great story-telling, it's no wonder that Hemingway's international reputation is as strong as in his homeland.

How is Ernest Hemingway considered an international artist?

If Hemingway is considered an international artist, it might have something to do with the fact that Hemingway never felt himself needing to be confined to one nation.  Hemingway understood early on that fame was akin to a "borderless passport," where the contours of national identity seemed to disappear.  Hemingway was an American driving Ambulances in World War I, wrote in Paris as part of the expatriated "Lost Generation," covered the Spanish Civil War as a correspondent, lived in Paris during the Allied Liberation, participated in safaris in Africa, and lived in Cuba and Key West before the Castro Revolution.  In such a varied existence, the only constant was Hemingway's fame and his notoriety along with a reputation that preceded him.  In this light, Hemingway was able to experience "the best of life" or wander from place to place in search of something resembling contentment.  Individual perceptions will differ on this point, but because of his own ability to travel the world over and create part of his literary and celebrity- like reputation on creating works in different part of the world, Hemingway can be seen as an international artist.