diff -u b/core/modules/user/lib/Drupal/user/UserConfigContext.php b/core/modules/user/lib/Drupal/user/UserConfigContext.php
--- b/core/modules/user/lib/Drupal/user/UserConfigContext.php
+++ b/core/modules/user/lib/Drupal/user/UserConfigContext.php
@@ -13,6 +13,11 @@
 
 /**
  * Defines a configuration context object for a user account.
+ *
+ * This should be used when configuration objects need a context for a user
+ * other than the current user.
+ *
+ * @see user_mail()
  */
 class UserConfigContext extends ConfigContext {
 
diff -u b/core/modules/user/user.module b/core/modules/user/user.module
--- b/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -1757,8 +1757,10 @@
   $langcode = $message['langcode'];
   $variables = array('user' => $params['account']);
 
-  // Get configuration objects customized for this user, that may be localized
-  // for the user's language if the locale module is enabled.
+  // Get configuration objects customized for the user specified in $params as
+  // this user is not necessarily the same as the one triggering the mail. This
+  // allows the configuration objects to be localized for the user's language if
+  // the locale module is enabled.
   $user_config_context = config_context_enter("Drupal\\user\\UserConfigContext");
   $user_config_context->setAccount($params['account']);
   $mail_config = config('user.mail');