Unfortunately, contrary to core ghost, the default theme will not be translated.
Maybe others have the need to have the Source theme translated and we could maintain a fork of Source with translations.
I’ve created a first release based on 1.2.0 which includes german (de).
Repo: GitHub - SourceTheme-i18n/Source: The default theme for Ghost
German strings: Source/locales/de.json at main-i18n · SourceTheme-i18n/Source · GitHub
The strings are where applicable based on the ghost strings: Ghost/ghost/i18n/locales/de at main · TryGhost/Ghost · GitHub