Not sure if you've used/seen this site, but it's a pretty comprehensive site with fabulous information (including a long list of bike rental shops) for self supported touring in Germany: http://www.bicyclegermany.com/index.html

Germany is such a bike-friendly country, and you're right, most everyone (in the old West anyway) speaks English. And a few phrases can help get you by if necessary for those who don't. I've self-supported toured in Germany (including East Germany when it was still East Germany), as well as other European countries. It's a great way to spend a vacation. I even had some panniers made out of cuben fiber for my last trip a few years ago. You can borrow them if you like.

You'll have a wonderful time.