Add a language switcher with English, Italian, Spanish and French

The interface is translated through @nuxtjs/i18n: strings live in
i18n/locales/*.json, routes are prefixed except for the default English, and
useLocaleHead emits the lang attribute, the hreflang alternates and og:locale.

The switcher is a native select rather than a custom dropdown: keyboard
support, the platform picker on mobile and correct labelling come for free.

Article content stays single-language — this translates the site chrome only.

frontend/shared/utils/format.ts

@@ -1,7 +1,7 @@
 /** Human-readable date for display; returns an empty string when unset. */
-export function formatDate(value: string | null | undefined): string {
+export function formatDate(value: string | null | undefined, locale = 'en-GB'): string {
   if (!value) return ''
-  return new Intl.DateTimeFormat('en-GB', { dateStyle: 'long' }).format(new Date(value))
+  return new Intl.DateTimeFormat(locale, { dateStyle: 'long' }).format(new Date(value))
 }
 
 /** Machine-readable date for the `datetime` attribute and structured data. */