The last instruction in https://github.com/Esri/geoportal-server-catalog/wiki/Localization states:
"Comment out locale: "en", in index.html under [Tomcat8]/webapps/geoportal and save the changes."
Though, in Geoportal Catalog 3.x, english locale is instead being forced in: [Tomcat9]/webapps/geoportal/lib/init.js:
window.dojoConfig = { locale: "en", async: true, parseOnLoad: true, packages: [ { name: "esri4", location: "https://js.arcgis.com/4.30/esri" }, { name: 'app', location: location.pathname.replace(/\/[^/]*$/, '') + '/app', } ] };