If you have visited the forum for the first time (as a guest) a language selection appears in the menu.
Here the language of the system is defined and also the default language for the topics and posts.
As a guest you will only see topics in the respective language... so either "German" or "English", according to the setting.
After registering, please go to the control center --> Settings --> General:
now change the language if necessary and (very important!) select both check-marks for the content languages:
regardless of your system language, you can now see *all* posts again.
Unfortunately, there is no other easier technical solution, but I hope everyone can handle it.
Now, when you create a new Thread, please enter the correct Language of the content you want to write. If you write it in english, please select English in the select box, otherwise select german.