I just watched with the greatest anger the following video about an unrepentant Nazi war criminal who has become the subject of considerable Neo Nazi worship. YouTube Link
As explained therein, the Germans can not prosecute him, because he can not be convicted twice for the same crime. He was convicted in France in 1949, but could not be extradited. The crime has since come under the statute of limitations in French law. The video says extradition was unstatthaft.
I want to know, as precisely as possible, on what law this inability to extradite was based and when the law was in force. I did some googling for „Auslieferung unstatthaft“, but found nothing.
Edit: The answers below explain very well, why he cannot be extradited now (this is my fault because in the comments I tried to argue there should be a provision for those cases) but I have not yet understood why he wasn’t extradited in 1949 („Auslieferung unstatthaft“).
[TLDR/tangent: the argument I am trying to make in the comments, but which doesn’t change the facts here is that non-extradition in 1949 creates the prerequisite for the french verdict to expire. Current law then uses the French verdict as an (understandable) reason for not extraditing, but overlooks the fact that the verdict was rendered moot by non-extradition. This strikes me as a completely predictable loop hole, easy to close by something like this: extradition can not be denied based upon a verdict that was rendered moot by non-extradition...]