Hi again @petranca,
Thanks for reaching back out. It is true that if you set your overall website settings to use Deutsch (Sie) – i.e. Formal German – then there are no translations available.
As a workaround, you can set your site language to just Deutsch, without the Formal modifier, and this will work. Translations for generic German are available.
If you are interested in translations for Deutsch (Sie), we rely on our user base to create translations for new languages. If you are interested in helping with this project, you can learn how to contribute translation files for the plugin here:
https://kb.mailpoet.com/article/169-translations-in-the-plugin