API Explorer 4 under Joomla and Windows/IIS gives fields like {{ ts('CiviCRM API v4') }}
The problem was caused by IIS Request Filtering Hidden Segments.  By default IIS has a hidden segment called bin.  API Explorer 4 attempts to get JavaScript from 

.../administrator/components/com_civicrm/civicrm/bower_components/google-code-prettify/bin/

Notice the bin folder name.  Removing the bin Hidden Segment cured the problem.  Security isn't an issue - the web site is only accessible in the local environment.