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.yml

@@ -52,6 +52,8 @@ services:
       NUXT_STRAPI_URL: ${STRAPI_URL}
       NUXT_PUBLIC_SITE_URL: ${PUBLIC_SITE_URL}
       NUXT_PUBLIC_STRAPI_URL: ${PUBLIC_STRAPI_URL}
+      # Absolute origin for the alternate-language links.
+      NUXT_PUBLIC_I18N_BASE_URL: ${PUBLIC_SITE_URL}
 
   caddy:
     image: caddy:2-alpine