I am opposed to any further fragmentation of the SFML userbase.
Yes, a German forum means a slight separation, but of a different kind than separating users completely. The forum is meant to be a place to ask questions regarding to programming with SFML in the native language, and not to replace the official forums. Feature requests, bug reports and also project demonstrations can and still should be done here, since it guarantees the communication with the developers. I stated that in a welcome posting at the German forum.
I think most german developers should be able to speak decent english.
Many do, but by far not everybody. Many people also just have a better understanding of things when they're written/explained in their native language. Especially younger people and beginners can benefit from that.
f not, they can maybe post here in german, and the germans here can help out
I don't think that is a practicable solution. This forum is intended to be English, so that most people can understand what's written here. If it'd be mixed with German, it can be mixed with everything, thus leading to a huge confusion.
Don't get me wrong here. I can fully understand your doubts about a German forum (or maybe call it a section). But I bet if there'd be a German sub-forum here, a lot of Germans would also use it. Currently there's nearly no activity at the German forum, and I'm still waiting for some progress so that it pays off. But what I disagree with is that the forum is a user separation. It's more an enhancement to help people in their native language. Since only concrete (project-wise) questions are asked in the several package boards, I think an extra forum is just okay.
But we'll see how that works out. I hope that more users will be visiting the forum and also take part in it, i.e. writing posts. If that won't happen, well, then there's obviously really not a need for it. But it's worth a try in my opinion.