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/app/error.vue

@@ -5,23 +5,20 @@ const props = defineProps<{ error: NuxtError }>()
 
 const isNotFound = computed(() => props.error.statusCode === 404)
 
+useHead(useLocaleHead())
 useSeoMeta({ robots: 'noindex' })
 </script>
 
 <template>
   <NuxtLayout>
-    <h1>{{ isNotFound ? 'Page not found' : 'Something went wrong' }}</h1>
+    <h1>{{ isNotFound ? $t('error.notFoundTitle') : $t('error.genericTitle') }}</h1>
 
     <p class="muted">
-      {{
-        isNotFound
-          ? 'The page you are looking for does not exist or has moved.'
-          : 'Please try again in a moment.'
-      }}
+      {{ isNotFound ? $t('error.notFoundBody') : $t('error.genericBody') }}
     </p>
 
     <p>
-      <NuxtLink to="/">Back to home</NuxtLink>
+      <NuxtLinkLocale to="/">{{ $t('error.backHome') }}</NuxtLinkLocale>
     </p>
   </NuxtLayout>
 </template>