Adding German translation

[deining]

This PR adds the German translation for the theme messages. It also comes with some minor corrections for a few English message texts.

While translating, I realized a few duplicate message strings:

5_togglesidebar_wording:    Collapse/Uncollapse sidebar
sidebar_collapsible_title:  Collapse/Uncollapse sidebar

6_toggletoc_wording:    Collapse/Uncollapse toc
toc_collapsible_title:  Collapse/Uncollapse table of contents

Are these duplicates on purpose? Shouldn't that be consolidated?

[jgazeau]

@deining , thanks for the PR.

Regarding 5_togglesidebar_wording and 6_toggletoc_wording they are actually for the sidebar and the table of content which are two different components (sidebar on the left and toc on the right) so they must be kept for now.