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CiviCRM has Smarty support which enables useful logic for contribution emails and similar things, but it is not enabled for email by default. You must edit /wp-content/uploads/civicrm/civicrm.settings.php and enable Smarty:
In addition, you should disable entities in CK editor. You can do that at Administer->Customize Data & Screens->Display Preferences. Then scroll down and click the “Configure CKEditor” button.
On the subsequent page, you will need to set entities to false.