I haven't really looked into the diplomatic fallout, though the issue did surface from time to time in the press. I know of nothing specific on this, but that doesn't mean anything (other than, maybe, the idea that it isn't something that has been written a lot about — but that doesn't mean it didn't exist).

As for "third-party nations" — the main non-Japanese victims of the bombs that come to mind are POWs (British, American, and Dutch), Koreans (laborers), and Germans (the Jesuits at Hiroshima, and maybe others). Of these groups, the ones most represented in American media are the Germans, who were featured quite prominently in John Hersey's Hiroshima, among other sources. The Koreans were by far the largest group of victims, of those groups.