Hello Lucas,
Its located in the language file included with the module. Just look for:
language\en-GB\en-GB.mod_s5_contact_popup.ini
inside this file look for:
S5_CONTACTPOP_SUCCESS="Your message has been sent. You will soon hear from us!"
Looking at your site it appears you have an older version installed of this module too. I recommend grabbing a fresh download and reinstalling this module as we updated it to use the Google captcha method.