Customize Login Window in Web GUI

I want to customize the Login GUI
LoginGUI.jpg
I have changed /usr/share/pve-manager/root/Index.pl lang en-->de
which gives me the german layout. Now I am looking for the place
where to change Sprache: English --> German.
It has no influence as all is in German anyway, but it makes no confusion to the person
logging in. I would definitely find the place with some time consuming effort.
But I am sure someone already did it.

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  Clik here to view. LoginGUI.jpg (15.2 KB)