Assuming you are using the most recent version of Lighthouse Studio.
From the main Toolbar, click the "Settings" icon. (Or click Compose + Survey Settings).
Then, select the General Settings tab. You will see a section of that dialog called "Navigation Buttons". Click the Settings button and you will find the place to edit the words on the buttons.
Also, pay attention to translate the error messages that respondents can see during the survey into German. These are found in the "Error Messages" tab within this same Survey Settings dialog.