That's one of the elements which is actually real (original photo: https://www.airliners.net/photo/IndiGo/Airbus-A321-271NX/528...) - the German flag (as well as the German registration and the MSN) are apparently added over the airline's paintjob until the plane has been delivered to the customer and is allowed to use its Indian registration (which is to the right of the Indian flag over the rear passenger windows, covered with a white sticker).