How to let the system talk not in English?

Typically home pages of a blog shows widgets in the sidebar: for searching, popular tags, archive. As I run my site primarily in German I want to display them in German. In Antimatter these values are set by e.g. SIDEBAR.SIMPLE_SEARCH.HEADLINE. How or where are their values set, how can they be translated - and is there already an existing translation? Also: the date of a blog entry is shown in English. And I know that in .../system/languages/de.yaml are the names of the months in German. How to access them?

Ok, in the meantime I've found the languages.yaml in the root folder of the antimatter theme. But how to translate the names of months is not solved yet.

There is a the template "partials/blog_item.html.twig". It represents the design of the blog entries in your overview. Maybe you can react on the Month there with some twig-coding.

There are also certain ways to upgrade twig to display other name of months for different languages. But I don't know if grav can handle this.