To change the label of the wishlist button, use this code for the non-logged-in user. Copy the code and paste it into the functions.php file.
add_filter('wishlist_button_save_login_label', 'wishlist_button_save_login_label_20201908', 5);
function wishlist_button_save_login_label_20201908($login_label){
$login_label = 'Custom Login Text';
return $login_label;
}