Hi jens,
it depends on the number of english speaking guests. On the last hackathon we had a few english speaking people. So it was a mixture of german and english.
Most developers are able to switch to english, so if you become part of one group there is a good chance, that the group will work in english. Also if there are english speaking people, the formal part of the event may be done bilingual or english only. But to be honest, right now, some of the prepared material may be written in german.
My wish is, that OpenEMS will be spread all over the world. So in the end I hope, that the hackathon will become a regular event and that we may have some parts in english and some parts in german sooner or later.
This depends on all the developers out there. Feel free to prepare a topic for the hackathon by yourself.
You are welcome.