Most scientific papers are now published in English, and I believe that this trend (n.d.-a) is good for international collaboration. Therefore I believe that blogs intended for scientists should also be written in English. The topics and discussions in German language science blogs are often similar to what we discuss in English, and it makes the audience much smaller if we restrict ourselves to a particular language (the same could be said about French, Italian, Spanish, Japanese or Chinese blog posts).
But there are good reasons to write German language science blogs, and the German science blogosphere is indeed very active. For those understanding German, I've listed some of the more popular blogs and blog hostings sites below: