feat: Adds language badges to tour overview table

layouts/shortcodes/README.MD

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+## Shortcode: Language badge
+
+Use the `lang` shortcode to mark whether a tour is in German, English or bilingual. It renders a small superscript badge next to the text, with details when hovered over or parsed by a screenreader.
+
+- German: `{{< lang de >}}`
+- English: `{{< lang en >}}`
+- Bilingual: `{{< lang deen >}}`
+
+For usage references check the [German](/content/de/39c3/_index.md) and [English](/content/en/39c3/_index.md) tour overview.

layouts/shortcodes/lang.html

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+{{- $code := .Get 0 | lower -}}
+{{- $validCodes := slice "de" "en" "deen" -}}
+{{- if not (in $validCodes $code) -}}
+  <span style="color: red; font-weight: bold;">Warning: Invalid or missing language code '{{ $code }}'. Use 'de', 'en', or 'deen'.</span>
+{{- else -}}
+  {{- $label := cond (eq $code "de") "DE" (cond (eq $code "en") "EN" (cond (eq $code "deen") "DE/EN" "")) -}}
+  {{- $title := "" -}}
+  {{- if eq $code "deen" -}}
+      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird zweisprachig stattfinden (Deutsch und Englisch)" "This tour will be bilingual (German and English)" -}}
+  {{- else if eq $code "de" -}}
+      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird auf Deutsch stattfinden" "This tour will be held in German" -}}
+  {{- else if eq $code "en" -}}
+      {{- $title = cond (eq .Site.Language.Lang "de") "Diese Tour wird auf Englisch stattfinden" "This tour will be held in English" -}}
+  {{- end -}}
+  <abbr style="font-size: .65em; vertical-align: super;" title="{{ $title }}">{{ $label }}</abbr>
+{{- end -}}