Add Scandinavian i18n (da/sv/nb) via DeepL pipeline.

Integrate new locale bundles, language cycling in the UI, SEO hreflang tags, and localized beta flyer HTML variants with scripts for batch translation and key validation.

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/utils/i18nLanguages.ts

@@ -0,0 +1,22 @@
+/** Supported UI languages (ISO 639-1, language-only). */
+export const SUPPORTED_LANGUAGES = ['de', 'en', 'da', 'sv', 'nb'] as const
+
+export type AppLanguage = (typeof SUPPORTED_LANGUAGES)[number]
+
+export function normalizeAppLanguage(language?: string): AppLanguage {
+  const base = (language ?? 'en').split('-')[0].toLowerCase()
+  if ((SUPPORTED_LANGUAGES as readonly string[]).includes(base)) {
+    return base as AppLanguage
+  }
+  return 'en'
+}
+
+export function getNextLanguage(current?: string): AppLanguage {
+  const active = normalizeAppLanguage(current)
+  const index = SUPPORTED_LANGUAGES.indexOf(active)
+  return SUPPORTED_LANGUAGES[(index + 1) % SUPPORTED_LANGUAGES.length]
+}
+
+export function isGermanLocale(language?: string): boolean {
+  return normalizeAppLanguage(language) === 'de'
+}