Assuming you only want to use an alternative templating language in your frontend themes then this should be fairly straightforward.
The key modification you will need to make is to replace the default frontend controller, the easiest way is to make your own frontend controller that extends the default Bolt one.
Then you will need to replace the
render() method in that controller with a method that has this signature:
protected function render($template, array $context = , array $globals = )
where template is the requested template name, and context and globals are variables to be passed to the template.
Once you have that, then you will need to replace the default Bolt controller with your own, you do that with a call to the app to replace the service:
$app['controller.frontend'] = $app->share(
return new Myapp\CustomFrontendController();
There might be a few other issues you run into but that should get you most of the way there.