Star Spangled Banner, first verse anyway, is probably closer to being a folk song outside the USA, in the sense that anyone who might know it well enough to sing it would have picked it up from hearing it. The same with the Marseilleise and the Internationale.

Tunes like Nearer My God to Thee and Abide with Me you pick up at funerals (or maybe football matches in the latter case), so they'd qualify too. And music hall songs, the choruses anyway, are also orally transmitted for most of us.