To change the subject of the newsletter mail , you need to override the i18n PLUGINS key in plugins
This is a configurable key with 4 parameters:
The default i18n kernel key is :
<message key="PLUGINS_NEWSLETTER_SEND_MAIL_SUBJECT">[{0}] {3} - {1}</message>
<message key="PLUGINS_NEWSLETTER_SEND_MAIL_SUBJECT">[{0}] {3} - {1}</message>
<message key="PLUGINS_NEWSLETTER_SEND_MAIL_SUBJECT">[{0}] {3} - {1}</message>