I have an '06 A6 Avant. When the MMI display starts I see I can choose any of three languages, English, Spanish or French. There is one button whose area on the display is blank. It's a GERMAN car. Why not German for the fourth choice???

 

Assuming you're in North America and your car was intended for the North American market, I'm going to guess it's because there is no location within North America where German is spoken widely. You have French speaking people in Canada, Spanish speaking people in Mexico (and U.S.), and obviously English.