Hey sebrousseau,
Thanks for posting in the support forum. In order to change the inline success message for the MailChimp plugin, you’ll want to edit the mailchimp.php file. To do so, head to the WordPress Dashboard, and then navigate to “Plugins” > “Editor”. There, use the drop down menu at the top right of the page to select the “MailChimp” plugin, and then click on the mailchimp/mailchimp.php link to the right to edit that file.
The success message can be edited on line 1456 or by searching for ‘mc_success_message’ in that file. The message will be the string inside of the .esc_html(__()), like this:
$msg = "<strong class='mc_success_msg'>".esc_html(__("This is the success message that you'll want to change", 'mailchimp_i18n'))."</strong>";
Once you’ve changed to message to what you’d like, click the “update file” button below the editor window, and the new message will appear when the form is completed.
Thanks again for posting in the forum, and if you have any other questions, please feel free to let us know. Have a great day!
-Gunter