WordPress doesn’t bundle any “templating engine” libraries like Mustache or Smarty, as far as I am aware, and frankly I have not been sold on the need for them. (Yes, I know its all the rage.)
Concatenate your own strings, use PHP native functions like ob_start or sprintf, or load your own templating library. I have seen plugins do that. I just recently saw one that loaded Mustache, and there is a WordPress plugin purporting to make Smarty available.