Johny.: Try to:
1. Log in and change your language in
https://www.gog.com/account/settings/personal (persistant change on server side concerning your account)
2. Go to main page
http://www.gog.com and change your language in dropdown in the footer.
3. Clear your browser cookies (might have impact when you enter site logged out)
4. Re-open your browser and check if the site is in Deutsch
Then we will know for sure if it's set by GOG servers for you.
Johny I've seen this happen only when I'm NOT logged in I believe. It has happen at my home and at my college on multiple PC's so the cookies would not be the same. If I just visit the site (when not logged in) the language comes up German on the first load of the main page. My college is like 30 min away, Eastern USA, I visited GOG for a few min to check out the releases today (did not log in at all) and I noticed the same thing it does at my home, main page was in German... pretty sure it's on your end and seems to be detecting my general location wrong sometimes.
I'll try this though... but it seems to be random, sometimes it loads in English correctly and sometimes it loads in German but like said, unless I'm mistaken, only when I'm not logged in. Meaning it's before it knows my language setting. I don't think I've never seen it come up in German if I've been logged in, which makes sense since my account is set to English.
If I refresh usually it will reset its self to English or I'll just manually change it. I also tend to clean my cookies/cache frequently.
Sorry I think it's German actually looked like Dutch based how it was spelled. Lol