There is no 'welcometocourse' string in the enrol_paypal plugin. The global 'welcometocourse' string has to be modified to get this working,
But if you only want to change the welcome string for PayPal enrolments, perpaps it is a better idea to change line 272 of the /enrol/paypal/ipn.php from
$eventdata->fullmessage = get_string('welcometocoursetext', '', $a);into
$eventdata->fullmessage = get_string('welcometocoursetext', 'enrol_paypal', $a);
Next, you only have to add 1 extra line in the /enrol/paypal/langy/en/enrol_paypal.php file:
$string['welcometocoursetext'] = 'You are enrolled using PayPal';