Followed the guide here: https://civicrm.org/blog/ayduns/language-switcher
Have English and Spanish (Mexico) installed.
When I switch the language by using a URL on an event link civicrm/event/register?reset=1&id=168&lcMessages=es_ES I still see the English UI elements untranslated, such as "Register".
I installed this extension: https://civicrm.org/extensions/language-switcher and have switched back and forth from English to Spanish (Mexico) and it does not show any CiviCRM backend element in Spanish.
What am I missing?