Contact form on contact page

Hi i'm building a french website. How can i modify the error lines (please enter your name, The reCAPTCHA wasn't entered correctly. Go back and try it again.... Your message was sent to us successfully. etc) in the form ?

I tried to modify template.contact.php in template folder without any success...

It would be the best to use poedit and create translation based on Simplicity.pot located in languages folder of main theme folder. There are many good tutorials about the way it should be done.
Bu You could also edit file:
/wp-content/themes/Simplicity/layouts/template.contact.php
and edit those strings manually.