[Gelöst] Assign myself as a translator (Admin)

Background of the issue:
I am the site admin and I am trying to assign myself as a translator on my WordPress site. The issue can be seen on the page: versteckter Link.

Symptoms:
When I try adding myself as a translator and type in my username, it is not shown in the list. I expected to see my username, but instead, I got nothing.

Questions:
Why is my username not appearing in the list when I try to assign myself as a translator?
How can I successfully assign myself as a translator?