Remove i18n, keep only Italian and rely on browser translation

The interface was translated in en/it/es/fr via @nuxtjs/i18n. Drop the
module, the locale files, and the language switcher: the UI is now
static Italian text, with translation for foreign visitors left to the
Google Translate browser extension instead of the app.

frontend/app/error.vue

@@ -5,20 +5,21 @@ const props = defineProps<{ error: NuxtError }>()
 
 const isNotFound = computed(() => props.error.statusCode === 404)
 
-useHead(useLocaleHead())
 useSeoMeta({ robots: 'noindex' })
 </script>
 
 <template>
   <NuxtLayout>
-    <h1>{{ isNotFound ? $t('error.notFoundTitle') : $t('error.genericTitle') }}</h1>
+    <h1>{{ isNotFound ? 'Pagina non trovata' : 'Qualcosa è andato storto' }}</h1>
 
     <p class="muted">
-      {{ isNotFound ? $t('error.notFoundBody') : $t('error.genericBody') }}
+      {{ isNotFound
+        ? 'La pagina che cerchi non esiste o è stata spostata.'
+        : 'Riprova tra qualche istante.' }}
     </p>
 
     <p>
-      <NuxtLinkLocale to="/">{{ $t('error.backHome') }}</NuxtLinkLocale>
+      <NuxtLink to="/">Torna alla home</NuxtLink>
     </p>
   </NuxtLayout>
 </template>