Although the userInfo.preferedLanguage is correctly filled in development, it stays null in production. This is really a pity because getting the preferedLanguage can be more than handy (ie: to set the correct locale for a date). There is a "strapi-admin-language" in the localStorage but it appears nowhere in the docs.
###Steps to reproduce:
Launch Strapi and open the devTools -> localStorage/userInfo under the Application tab
import { auth } from "@strapi/helper-plugin";
const user = auth.get("userInfo");
console.log("user", user);