After a long break from kirby I’m coming back and feel like I’ve forgotten half the stuff, so please excuse my question: Is there a way to set the default panel language for a new user? Right now it defaults to English, and I would like to set it to German.
No, you cannot define a default language for a new user, it always defaults to the current default language.
You should be able to set the default (preselected) language for new users with the panel.language option.
Haha, helps to read the docs …
I didn’t know if that affected the select box, but after some digging through the code, it turns out it does. 
Thank you you both 