So the user MUST select German or English as his language of choice. But when I check the new user entries, some of them have no langage selected?!
I've no idea, this is for the most part handled by Joomla. I'll be reviewing the user params usage for CB 1.10.0 as I'll be updating it with a parameter to hide unwanted user params fields. Does this work fine if setting in backend through CB > User Management?
Same happens with subscription plans. The user has two radio buttons, one is preselected and it is not possible to disable all radio buttons (2 plans by default visible). Nevertheless, I have users in my database with no plan selected.
This can happen if they register, but cancel their basket before paying. They'll be registered, but with no subscription. The other way it can happen is they're not using CBs registration, but are using Joomlas.
Do you have any idea if and how a user can bypass the registration form? The users in question are definitely not spam-bots etc.
They may not be bypassing it. They may just be cancelling the basket and deciding not to pay. This is normal as some users may change their mind then just leave your site.