Just a note, this forum uses standard software as well. The software itself is relatively new (2013) and based on modern principles and is 100% open source. GitHub - discourse/discourse: A platform for community discussion. Free, open, simple.
It is also used widely across the internet, so it’s not exotic software. But it’s also not one of these outdated PHP message boards.
Mix of German and English, yep, that was discussed frequently 
- currently that’s the best solution we have. If you have a better idea how to handle this, please feel free to propose something here: Wieso gibt es kein rein deutsches Forum oder Support and here The language thing.