Is it advisable to attend a conference in Germany with talks by German presenters in English if you don't speak German?
Here are some claims that I think you will agree are true:


Anyone who attends a conference where all the talks are in English can understand spoken English reasonably well
You don't live in Germany and are considering attending. You are probably not the only person considering it
Anyone willing to give a technical presentation in English can probably both understand and produce spoken English reasonably well


I believe these add up to you being able to find people for "hallway conversations" in English pretty easily. You might have some lonely lunches if you find yourself at a table with people who all speak German and exclude you. I have this problem with people who all discuss (in English) something I don't understand at all and don't want to learn (eg how to install Exchange on a server.) I generally try to prevent this by striking up a conversation with a fellow attendee at the end of the session that is right before lunch, and suggesting we go to the lunch line together.