REST API: Add en_US to the locales enum.

Props ocean90.
Fixes #38528. 


git-svn-id: https://develop.svn.wordpress.org/trunk@39096 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

@@ -1069,7 +1069,7 @@ class WP_REST_Users_Controller extends WP_REST_Controller {
 				'locale'    => array(
 					'description' => __( 'Locale for the resource.' ),
 					'type'        => 'string',
-					'enum'        => get_available_languages(),
+					'enum'        => array_merge( array( 'en_US' ), get_available_languages() ),
 					'context'     => array( 'edit' ),
 				),
 				'nickname'    => array(