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 ::before
or ::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.
To actually use a completely separate, translatable alternative, the Flarum javascript extension API would have to be used. If you want to go that route, you could look at how the Tags extension customizes the background color of the button. Something very similar can be used to modify the children
property of the button component.