I believe, nothing.
Nationhood and by corollary, corporations in law last forever, unlike human personhood. So if the Germans decided to suspend payment (which was token, ceremonial and NOT resumed until AFTER reunification) the contract is still valid. Just like when Hitler suspended payments in 1933, which also did not start a war. The real problem was that the UK and France borrowed to fund WW1, and had to make payments to the Americans. No reparations, no money to the Americans, unless the Americans were generous and allowed default.
Within any industrialised nation there is a vast bureaucracy and civil service which outlasts governments to prevent leaders from doing stupid things. Barring any sort of purges or true revolution (even then some), treaty is law and nations respect law. There may be blips in history where law is suspended, but unless the country is completely cut off from the world and the entire educated class liquidated, lawyers and civil servants will recommend what is sensible and nudge leaders to respect treaties. If a nation wants any connection to its past, it will respect treaties and laws if only in token. The most likely excuse is reunification is too costly so we will "pay later"... maybe in the 21st or 22nd or 23rd Centuries. After all, you can always inflate the currency, make up excuses, etc., because you aren't limited by a human lifespan. And you can always issue more bonds, a reverse bond ladder. Of course a demagogue can be elected or become Prime Minister or Supreme Leader or whatever the title... but government lives on, far beyond any hot heads or issues of the moment.
http://www.nytimes.com/2014/12/28/world/that-debt-from-1720-britains-payment-is-coming.html?_r=0
Long answer short, it wouldn't happen... once everyone who cared was dead and in the grave, no more emotions or politics, and the bureaucratic machine of government takes over and will pay just to maintain goodwill... when 10 million is worth 1 million... but it will be paid.