Not sure what prompted the dog to bite you. I know some dogs just plain don't like certain things about people (ie; male, female, facial hair, purfume, etc). There's alot of reasons animals bite people, and sometimes you just never really know why.

I don't know about the situation in Germany, but here Rabies is always a big concern. We have to quaranteen animals for 10 days if they bite someone to be sure they don't have Rabies. Since you don't know where this dog and it's owner live the animal can't be observed for a 10 day quaranteen, you would need to take the rabies postexposure shots to be sure. Rabies is almost always fatal, it's not worth the risk when it's preventable.

http://www.cdc.gov/rabies/exposure/a.../domestic.html

I do hope you are able to find the dog, so you don't have to go through the shots. Good luck.