language preferences is missing in "my profile"

Hello!
I changed my profile language a few weeks ago into English.
Now I would like to change it back to German, but the menu item "Language Preferences" is missing in "Personal Preferences".
Can you help me?
In the official getting-start-guide the menu item is referenced to:

If I open my own profile page, I do not have this menu item anymore ...

Do you have any hints how to change the preferenced language?

Thanks for helping,

Martin