Index: src/wp-admin/includes/user.php
===================================================================
--- src/wp-admin/includes/user.php	(revision 39243)
+++ src/wp-admin/includes/user.php	(working copy)
@@ -100,8 +100,10 @@
 			$locale = sanitize_text_field( $_POST['locale'] );
 			if ( 'site-default' === $locale ) {
 				$locale = '';
+			} elseif ( '' === $locale ) {
+				$locale = 'en_US';
 			} elseif ( ! in_array( $locale, get_available_languages(), true ) ) {
-				$locale = 'en_US';
+				$locale = '';
 			}
 
 			$user->locale = $locale;