People in the military charged with a crime in Germany or any other country with a US military base are tried by the US Military court. This process is clearly defined in the treaties allowing the US military base in the country in the first place. If any country disagrees with this they can make the US military leave. But any country taking this action can expect any mutual defense agreements or military support to also be cancelled. The US military bases in Europe and Asia are not there to protect the US they are there to provide trip wires that will guarantee US military support if the host country is invaded. Can't really see any other country doing the same thing for the US and it is for that reason the US should remove their military bases and let the fun begin.
This is not accurate for a couple of reasons.
First, the 'treaties' you are referring to are actually called SOFAs, or Status of Forces Agreements. And generally speaking, US military personnel who commit crimes against host nation citizens/institutions can expect to face the host nation's criminal justice system.
Second, they are almost always *not* defined in the treaties establishing the bases; they are negotiated separately. However, failure to reach an agreement can lead to (or else accelerate) bases not being established, and/or troops being removed. This change in status for US military personnel in Iraq was one sticking point a few years back.