German Guy here. There is no law against legaly bought games frome "different" sources as long as you´r an Adult (18 years old) and don´t spread it to minors that wouldn´t be able to buy it. It´s just that "erotic" games and "certain symbols" (don´t need to fill in here what kind, do i?) are not allowed in the Steam Store (they don´t have a german goverment approved way of testing that you´r over 18 years old). Only other Problem i see is the "dlc" stuff we don´t have access to the "Store" page and can´t buy dlc´s. :( is there maybe a why that you make them avalibale here?
So Short Version:
Germans can activatet your games, its legal and have fun. Cya
Anleitung für Deutsche: Steam öffnen, Reiter Spiele, Ein Produkt bei Steam aktivieren, Key im neuen Fenster eingeben (copy and paste) und ne Taschentuch box bereit halten oder wahlweise socken.