Ah now I understand the issue
With the dropdown you change the frontend website dimension to german, english or what ever.
But the create dialog is a component of the backend ui. So what you want is to change the backend ui language.
You can do this in the user module.
So the language label you configure is for making the backend language available for others.
For instance when you have german editors or frensh editors.
The dimensions you are configured are for the nodes that are rendered in the frontend end.
So you go to the user managment module and choose your user.
And there you can chnage the backend language to german for instance.
Save that and go back to the content module.
Then the create dialog should show the german label