Likewise the ballads per se may not be that old, perhaps only in any numbers 15th century, but many of their stories are many centuries older.

It's not that difficult to conceive of them being translated from one language to another in great numbers pre-print and pre-mass entertainment. They would have been a highly valued commodity.

I've seen examples even of a ballad from Germany being translated westwards and being found in great numbers in Brazil within half a century.

A good example of story into ballad is the Bramble Briar. Decameron 'Isabella and the Pot of Basil' (without the pot) as a story translated into English 17th century, used as the basis for a ballad in Bristol c1750, enters oral tradition all over English-speaking world, 19th century onwards.