I have already spoken to Muntasir on Facebook. I was able to localise the problem:
The backend language is set to German. However, if you want to create a menu link and select an SPPB page, a white page is displayed. Umlauts are also not displayed.
https://prnt.sc/X8zLAm3JOyRB
If you change the backend language to English, it is again possible to select SPPB pages for the menu links.
I have uploaded the language files from Paul and renamed them for the template. Helix is still not translated.
Some language files must now be adapted by JoomShaper ;)