Milka is largely produced in southern germany (Lörrach) and Austria. As they source their milk locally, it would generally contain, in some (or even all) cases probably exclusively "Alpenmilch".
As others have pointed out, "Alpenmilch" is not a protected term like "Champagne", so useage varies wildly. Milka uses the definition of Alps+Alpenvorland, which is comprised of the southernmost districts of Germany. They at least used to get Milk from the Company OMIRA which sits in Ravensburg - definetly not a very alpine region, but also not too far off.