I can get even more compliacted on the continent where not only people and songs move but borders as well.
Is an Alsatian folksong (in Alsatian, a German dialect) a French or a German folksong? Here, the solution is easy, for both counties involved would call it Alsatian, but Alsace never was a nation, so such a song would have no "nationality".
Annie of Tharaw (title of Longfellow's English transalation): The song was originally written in a local dialect (Low German with some Slavian words), translated centuries ago into High German. The monument for Annie stands in Lithuania, Tharaw is now in Russia.

Wolfgang