Altering User Registration Message I need to take control over and change the flash notification a user gets when they register, but administrator approval is required. 

I want to alter this...
"Thank you for applying for an account. Your account is currently pending approval by the site administrator. In the meantime, a welcome message with further instructions has been sent to your email address"

https://api.drupal.org/api/drupal/core%21modules%21user%21src%21RegisterForm.php/function/RegisterForm%3A%3Asave/8.2.x

It states...
'Normally this method should be overridden to provide specific messages to the user and redirect the form after the entity has been saved.'


I'm trying to figure out what the Drupal 8 way of doing this is. 
Should I be trying to "alter" the message with a hook? 
Should I be overriding a service? (though from my understanding this
is not service). 
Should I be extending this? 
Or is there some other best practice way of overriding this?


I'm learning Drupal 8. So I'm not entirely sure the "right" way to handle this particular case.
Brandon's answer is good for altering the email messages. 

If you are looking to alter the status message that pops up afterwards, I found this answer helpful: https://stackoverflow.com/a/38797543/963866