A lot of foreigners agreed to serve as combatants.
Indeed they did!.. In fact, the Waffen SS had several divisions of troops who were all volunteers from "foreign" nations, including the Dutch, Danes, Norwegians and several others, they even had a Russian Cossack cavalry unit, and one of Muslims, (who wore a fez as part of their uniform).
Those were not the "
Hiwis" they were actual combat troops.
Franco's Spain, virtually wrecked and broke after their civil war, maintained "official" neutrality, but did contribute a division of troops the German war effort as their only means of paying back the Nazi help Franco got during their civil war. The Spanish "Blue Division" fought on the eastern front and was destroyed at Stalingrad, (IIRC
)
I think GIs finding some training ammo in captured fighting positions and assuming it was being used against them, was a reasonable assumption. Hell, there may have been times when it was, but I can't see any soldier (of any nation) intentionally doing that if he had anything better to shoot the enemy with.