Arthur Chin, America’s first WWII ace, flew for the Nationalist Chinese, and (unlike the AVG “Flying Tigers”) actually fought the Japanese before the US entry into the war. Some Swedes flew with the Finns, and John Robinson (US) and Hubert Julian (FR) flew for Abyssinia against the Italians in 1935. What non-Spaniards flew in the Spanish Civil War, aside from the Condor Legion?
Well, in the Spanish Civil War, you need to look first at the International Brigades:
Mostly these were infantry as I understand them, but the ‘Republic’ of Spain did have an airforce so it’s likely they had trained pilots flying some of those. Unlike China in 1940, the would be allies would not dare support the Republicans given how the Spanish Civil War started, with Marxists and Syndicalists looting shops and burning churches and the worrying influence of the Soviet Union. So actual aircraft from those countries were not gonna be forthcoming.
Thing about aircraft is that they’re very expensive and logistically intense and minor powers just can’t afford big air forces
The French sent over 70 mostly obsolete aircraft, but the Soviets sent their newest and best Polikarpov fighters and Tupolev bombers, with about 777 pilots and instructors (which proved inferior to the German models that arrived with the Kondor Legion).
But the very nature of a movement made up of competing ideologies in loose confederation meant that the Republican Air Force (“La Gloriosa”) was never able to gel into a cohesive and decisive arm, and their tactics could be likened to six years-olds playing soccer, with everyone running after the ball and nobody playing defense.
Most of the Russian pilots who survived were later killed either in the purges or the war. American who served or even observed the Spanish Civil War, e. g. the writer John Steinbeck, were rejected for service during the Second World War. Many of the French and Spanish fighters who sought refuge in southern France became instrumental in the resistance or fought with the French volunteer brigades.
In addition to foreign fighters in the Spanish Civil War for both sides on the ground and in the air, there was an American flying squadron that fought against the Soviets in the Polish-Soviet War (I know you’re asking about the 30s, but hey between two wars). During the Second Sino-Japanese War, Colonel Chennault and his “Flying Tigers” served in the Republic of China’s Air Force against Japan.