The message is generated from a resource string inside of the ~/CMSResources/CMS.resx
file.
<data name="notifications.subscription.successful" xml:space="preserve">
<value>You have been successfully subscribed.</value>
</data>
You can edit that file or you can use the Localization app in the admin tool. See this documentation https://docs.kentico.com/k12sp/multilingual-websites/setting-up-a-multilingual-user-interface/working-with-resource-strings