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.

docker-compose.dev.yml

@@ -18,3 +18,4 @@ services:
     environment:
       NUXT_PUBLIC_SITE_URL: http://localhost:3000
       NUXT_PUBLIC_STRAPI_URL: http://localhost:1337
+      NUXT_PUBLIC_I18N_BASE_URL: http://localhost:3000