[Gelöst] The WPML plugin language feature is not working.

Background of the issue:
I am trying to translate content using the WPML plugin, but some contents are not translated. The issue can be seen on the page: versteckter Link

Symptoms:
The WPML plugin language feature is not working, and some contents are not translated.

Questions:
Why are some contents not translated using WPML?
How can I fix the WPML language feature not working?