XHP templates in Hack wind up being really nice, letting you create composable XML tags, defining attribute types, etc. And they look a lot like JSX, except with extending classes. I suppose that counts as an "internal DSL."

    //<csrf-form>(...)</csrf-form>
    class :csrf-form extends :x:element {
      attribute :form;
      use XHPHelpers;

      protected function Render(): XHPRoot
      {
          $token=get_csrf_token();

          return <form>
            <input type="hidden" name="token" value={$token}/>
              {$this->getChildren()}
          </form>;
      }
    }

More boilerplate than Twig, but no string concatenation.