Luckily, we have just the right organisation for that called the Bundesanwaltschaft ("Federal Prosecutor's Office"), who are currently looking into the revelations in a so called "preliminary examination phase". One which will lead to an actual investigation ... hmm well, maybe.
The Federal Prosecutor General Range was interviewed a few days ago by a public radio station; and said the following(via) (my translation):
But we also need facts which allow us to determine how significant the downsides are and if these are so severe that the prosecution has to stand back. According to our Constitution*, the foreign policy concerns of the Federal Republic of Germany and therefore the security of the citizens against attacks from the outside - against animosities from the outside - is of very considerable importance; and the lawmakers have taken this into account in the provision you mentioned.
Well, the "provision" is § 153d StPO and it states that the Federal Prosecutor General "can abstain" from an investigation if there is "the risk of a severe disadvantage" for the Federal Republic. His answer pretty much sounds like he has already made up his mind, but does not want to take responsibility for his own decision; that or he actually doesn't know that it says can, which would be pretty let's say hmm sad.
So what might be those severe disadvantages that Germany could face? Well, the US could start considering Germany as a third class partner; and in case of a severely damaged friendship they might even start spying on German citizens and firms. But we would have reached a cold-war-level relationship if they even considered eavesdropping on Merkel's private phone. Well, guess what... So, does he fear drone attacks or something? Nuclear annihilation to prevent terrorism (TM)?
A more important consideration for the Federal Prosecutor General should be if large parts of the German population start believing that "justice is done". But, who cares about the sovereign anyways, huh?
* this might have looked like a formal distinction just a year ago: Germany does not have a real constitution Germany has a "Grundgesetz" "Basic Law". It was planned to switch to a constitution once Germany is reunified, which, well, happened over twenty years ago, but that whole constitution thing seems to have been forgotten. Today, we have to ask ourselves serious questions about the souvereignity of Germany, so this is becoming important.