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/package.json

@@ -11,6 +11,7 @@
     "typecheck": "nuxt typecheck"
   },
   "dependencies": {
+    "@nuxtjs/i18n": "^10.6.0",
     "marked": "^18.0.11",
     "nuxt": "^4.5.2",
     "vue": "^3.5.41",