Translation not working

I have problems with the translation of my ghost page. I use the official Starter pack and the Source theme. In the settings of my site the Site language is set to ‘de’, because my site is in German. Under Labs the Portal translation feature is activated. But most of the navigation and buttons are still in English. What am I missing?

Here is my site:

As you can see in the upper right corner for example there is a link ‘Sign in’ and a button ‘Subscribe’ which I would like to be in German.

I found the locales under Ghost/ghost/i18n/locales at main · TryGhost/Ghost · GitHub

But I don’t know how to use them. I found this Ghost Handlebars Theme Helpers: translate but I dont’t think this works with Starter?