I would guess that none of the responsible people here noticed that this site violates German law (sort of) or they didn't care, since this store isn't in contact with a large part of the German population.
By the way, this isn't a matter of legal conflict, this site, or any site outside of Germany, doesn't need to take the German law into consideration... the worst that could happen is that GOG will be removed from the search results here, resulting in decreasing profits for them.
So, in conclusion, offering a German translation will most likely attract more German customers, resulting in a higher interest from the government. So, by locking the games available here and forbidden in Germany, GOG possibly tries to evade the mentioned consequences.
Though this is of course still only a theory, it is the only one that I could come up with and that makes sense to me.