Thanks for the reply, Simon! We were thinking of using plain .po-files too, however, we couldn't find the matching "src" folder in our installation, do you have any more details where it or the .po-files may also be located? What component are you talking about? For Postorius and Hyperkitty you should be able to create the files needed according to this https://docs.djangoproject.com/en/2.0/topics/i18n/translation/#message-files First run "django-admin makemessages -l de" to create the .po files and after editing "django-admin compilemessages -l de" to compile them. You should be fine doing that in your production django-project. Otherwise you can clone the frontend projects and run the commands inside the project folders. (src/postorius or hyperkitty/). I know the second option works, since that's what I'm doing (I'm deploying from the git heads) but doing that in the production projects (where manage.py is located) should generate the same files. It might generate files for dependencies as well...
As for core, I only know how to translate the templates. I have no idea how the messages included in the source code should be translated.