Back when Mercedes bought Brawn I'd certainly have an educated guess that having at least 1 German driver was high on the wishlist. Either that or Brawn suddenly fell out with Jenson or was overruled from above on the driver choice. They did say that having a German driver wasn't a necessity but I suspect it was as good as one. Any 'National' team needs to be seen as supporting their own drivers in some way. The only real exception to that is Ferrari but anyone that drives for them as good as becomes Italian anyway.