How do I change the language on the register page?

First, I would like to know if I can change the language on my registration page. Currently, the page has the following language "Registering for this site is easy, just fill in the fields below and we’ll get a new account set up for you in no time." I would to customize it with the organizations name; however, I am not sure which file do I edit.

Secondly, is there a way to test the Membership Pro plugin by using PayPal Sandbox. If so, how do I do that.

Thanks!