feat: add multilingual support with German and English

Implement complete multilingual support using Hugo's multilingual mode with German (de) as default and English (en) as secondary language.

Changes:
- Configure Hugo multilingual mode with language-specific parameters
- Reorganize content using language suffix pattern (filename.de.md, filename.en.md)
- Translate all content to English (home page and CV page)
- Implement language switcher that maintains page context
- Add language-specific navigation menus
- Update templates to use language-aware URL functions (relLangURL)
- Fix navbar brand and Home links to respect current language
- Add translationKey to content frontmatter for explicit translation linking

URL Structure:
- German: /de/ (home), /de/cv/ (CV page)
- English: /en/ (home), /en/cv/ (CV page)

Key Implementation Details:
- Use filename.{lang}.md pattern instead of {lang}/filename.md subdirectories
- Always use relLangURL for internal navigation, never .Site.BaseURL
- Menu URLs must include language prefix when defaultContentLanguageInSubdir = true
- Added translationKey to frontmatter for proper translation discovery

Documented 5 critical bugs and their solutions in design.md for future reference.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

layouts/partials/header.html

@@ -1,7 +1,7 @@
 <header>
   <nav class="navbar navbar-expand-lg navbar-light bg-light">
     <div class="container">
-      <h1><a class="navbar-brand" href="{{ .Site.BaseURL }}">{{ .Site.Title }}</a></h1>
+      <h1><a class="navbar-brand" href="{{ "/" | relLangURL }}">{{ .Site.Title }}</a></h1>
       <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
         <span class="navbar-toggler-icon"></span>
       </button>

layouts/partials/nav.html

@@ -1,12 +1,30 @@
 <div class="collapse navbar-collapse" id="navbarNav">
   <ul class="navbar-nav ms-auto">
     <li class="nav-item">
-      <a class="nav-link{{ if .IsHome }} active{{ end }}" aria-current="page" href="{{ .Site.BaseURL }}">Home</a>
+      <a class="nav-link{{ if .IsHome }} active{{ end }}" aria-current="page" href="{{ "/" | relLangURL }}">Home</a>
     </li>
     {{ range .Site.Menus.main }}
     <li class="nav-item">
       <a class="nav-link{{ if $.IsMenuCurrent "main" . }} active{{ end }}" href="{{ .URL }}">{{ .Name }}</a>
     </li>
     {{ end }}
+
+    {{/* Language Switcher - Show only alternative language */}}
+    {{ if .IsTranslated }}
+      {{ range .Translations }}
+        <li class="nav-item">
+          <a class="nav-link" href="{{ .RelPermalink }}">{{ .Language.LanguageName }}</a>
+        </li>
+      {{ end }}
+    {{ else }}
+      {{/* Fallback to home page if no translation exists */}}
+      {{ range .Site.Languages }}
+        {{ if ne $.Language.Lang .Lang }}
+          <li class="nav-item">
+            <a class="nav-link" href="{{ .Lang | relLangURL }}">{{ .LanguageName }}</a>
+          </li>
+        {{ end }}
+      {{ end }}
+    {{ end }}
   </ul>
 </div>