Most all Kentico web parts have their text I'm a localization string that you can overwrite just by creating a localization string matching it. Kentico prioritizes the custom localization string over the ones found in the.resx resource file in Kentico folder.
To find the right resource string, check the webpart layout tab and "create a new layout" (webparts - edit - layout tab - new). Then look at the code (the ascx that shows) and you should see the message localized control, copy the localization string and create a localized string with as matching key.