I'm not sure it's that easy to change it per tag.
Linguist allows to easily change the text, but it will be changed on all pages that show the Start discussion button.
You could use custom CSS and the
::after pseudo-elements to insert the alternative text. This wouldn't be very accessible to assistive technologies (screen readers, search engines, etc.) but since the original text is still there and would just be hidden it might not be a bad compromise.
children property of the button component.