Add German translations and automatic language selection

[ClemensElflein]

Adds German versions of all 83 content pages, including the homepage, documentation, community page, and release notes, with localized interface text, gallery captions, and search results.

On the first visit, the site selects the first supported language in the browser's preference list, falling back to English. Manual language choices are remembered. Automatic switching uses the matching page and preserves query parameters and anchors; pages without a translation remain in their original language.

Language links, canonical URLs, reciprocal hreflang entries, and sitemaps support both languages. Shared images and downloads stay in the English bundles, and archived documentation retains its versioned routes and noindex rules. The German copy has been edited for natural phrasing and clear instructions.

Validation:

• All 83 translation pairs pass integrity checks for executable examples, links, references, includes, dates, authors, and captions.
• All 14 automatic language-selection checks pass, including missing translations, saved preferences, and unavailable storage.
• Hugo 0.111.3 extended production and archive builds pass; generated-page checks cover 162 sitemap pages and 93 redirects.
• Browser checks confirm German detection and persistent manual selection of English; the required EXIF check passes.