What's a proper title when I started a post-doc position but not received my PhD degree yet?
I would use research fellow as your title in English and in German use whatever your contract says or research fellow as well. That leaves out the slightly awkward postdoctoral part. 

In general, unlike a doctorate, using the title 'postdoc' is not something that people in academia worry a lot about (and outside academia many people will have no idea what it means). So using postdoc to describe your role wouldn't be outrageous even though linguistically it seems wrong because of the implied doctorate.