Vaughan Williams, Aaron Copeland, Smetana, Virgil Thomson, Brahms and many others have interpolated folk music themes in various of their serious compositions. They have sometimes hinted at the music, but more often, it seems to me, have used much of the music itself and turned it into something different though recognizable.
On the other hand, I hear the opening notes to 'HaTikvah', the Israel national anthem, in a brief, recurring passage in Smetana's 'Ma Vlast'.
And the notes for 'Tho' April showers may come your way' (the old Al Jolson hit) can be head one time in the Winter movement of Vivaldi's, 'The Four Seasons'. Was Louis Silver, the composer, aware of Vivaldi?
I'll bet the real musicians and musicologists out there can come up with myriad {not a myriad of} other, better examples.