@mirjam : The prev post / next post are translation ready. You will need to download and install Poedit
Open Poedit and click on Create New Translation => Browse and Select zubin.pot file from the languages folder which is located inside the parent theme folder Zubin.
Click Open => Select the language as German.
Now, you will see a list of translation ready strings and Translation text area where you can add the required translation. Search for the strings prev post and next post and add the desired translation. When you are done translating the strings save the file to the languages folder. You will find two other additional files woth .po and .mo extensions inside the language folder of zubin.
Let me know if this helps you out.