Okay, retried because of your suggestion and now, when translating the buttons, it came up that I should push volume-up. Did this and now I have the international rom again, that I had before, but user data is wiped.
For the ones of you in need of doing the same:
1) Push Power and volume down at the same time
then this comes up:
(I have added a translation overlay for the button)
2) Select the second/middle Chinese language button with volume keys - then hit power
this brings up:
(I have added a translation overlay for the headline)
3) Push volume up and wait. After some minutes, it should bring up the regular lock screen and you can reconfigure your phone.
The third step is where I always tried to hit the Chinese language button because I couldnt translate it. Now that I managed to translate full headline, I realized that this is probably only the escape button and you need to push volume up.
Thanks for all your support!