Its just a number. Everyone knows that they have no intention to pay it back. Even if you add 10 more zeros it will have no impact since its worthless.
0*19600000000000 = 0
0*19600000000000000000 = 0
You can add as many zeros you want and it's still just worth what you are paying back, in other words 0
That is the spirit. There is no way the debt will be paid back. The debt will be inflated away, as many governments do in the past centuries.
Other countries shouldn't dare to try and collect on the debt, though, as it will damage the global economy severely. Even though the currency's just a bunch of paper that they print.
They probably use the money they borrow from other countries, that is worth less when they borrow it a couple years later, to pay the old debts.
Rinse and repeat.