Organization admins can change the default twitter share message for all petitions by going to CMS > Content > Social. The toolset automatically appends petitions' links, so the only text that you should enter here is the message that precedes the petition URL.
While this text can be overridden at the petition level (details below), the text entered in CMS > Content is the default text that's used for all petitions on the platform. Changing this text will affect the Twitter share options included on petitions' signatures pages and petitions' after-signing pop ups.
If you want to change the twitter share language for a single petition, navigate to the petition and click Admin. From here, go to the Admin Settings tab. After expanding the Social Media section, you can enter a petition-specific share message into the Twitter Share Message box. For more information about the other text boxes included in the admin settings tab, see: https://controlshiftlabs.zendesk.com/hc/en-us/articles/235204228-Customized-Social-Sharing