That would be the sendmessage.php language file.
Basically, if a language string is used in many places, it will be in the generic.php language file. Things like the navigation menu items, are in here, since they are on all pages.
Otherwise, if you see a string that you need to change on a certain page, just look in your address bar. You'll see a ubb=somescript. So, you'd look for the somescript.php language file.
