Pic 3: this looks like an app that is related to accessibility settings. Just above ABRECHEN and OK at the bottom of that pic, you appear to have ticked a box - untick it.
Once you've done that, switch your phone off then holding in the VOLUME UP and the other button halfway down the LHS, simultaneously press the POWER button, and hold them in until you see the startup screen (all 3 buttons are held together). This let's you do a hard reset. Hopefully, this starts up in English. If its in German, use your volume down button to scroll onto the line containing the word RESET. Press the POWER button to confirm this. If the phone restarts in German, login to Google, as per normal. If you set up the phone, let me know, and I will do a separate walk through with you, so that we get the default language to English.