There is a longstanding bug in the DVD::rip package that prevents it from ripping the subtitles if you are ripping from a directory instead of a mounted DVD.

This patch works for me:

+++ Project.pm 2012-02-01 11:59:58.359790393 +0100
@@ -376,6 +376,11 @@
         = $self->get_mount_dir_from_mtab( $dvd_device, "/etc/mtab" )
         || $self->get_mount_dir_from_mtab( $dvd_device, "/etc/fstab" );

+    # If no mount point was found, assume that it is an image dir
+    if(!$dvd_mount_point) {
+        $dvd_mount_point = $dvd_device;
+    }
+
    return $dvd_mount_point;
 }