Now I see the reason why it happened to me. I pressed the “original version” button and it was left like that. As the original thread keeps to be in English and now I saw the German comment, I thought the translation failed or something. But I understand German enough to respond. So I quoted the original.
Only today I realized I had the “original” turned on and switched it back! So it was partly a UX problem. Maybe a warning “You are quoting the original version, are you sure?” would help.