Moment JS has some Localized formats which you can use as format values. You can check them here: https://momentjs.com/docs/#/displaying/ (section "Localized formats")
I have those screens translated in a Plone 5.1.5 where I am working right now:
There must be something wrong in the pattern configuration or in your Plone site not to load the translations. You can check the pattern configuration here, I see a language list and perhaps your language is not there and dates are not shown correctly: https://github.com/plone/mockup/blob/master/mockup/patterns/moment/pattern.js#L91
On my Plone 5.2 instance it seems to work before switching to english and back to german
Correction: upon login, the text is in german. Reloading the same page changes the text to english. This behavior then persists even after logging out (i.e. must start a private browser session to observe).
Correction 2: forcing a reload with ctrl-shift-r reverts the text to german