"Edit membership in group @group" not translatable

When editing a group membership via the UI (using group/node/%nid/admin/people/edit-membership/%membership_id), I get the title "Edit membership in group MyGroup", even when I use German language. I tried to translate this string, but no success.

The String is located in og_ui.module, line 64:

$items['group/%/%/admin/people/edit-membership/%og_membership'] = array(
  'title callback' => 'og_ui_menu_title_callback',
  'title arguments' => array('Edit membership in group @group', 1, 2),
  ...

But if I go to /admin/config/regional/translate/translate, I can't find this string there. So it is not translatable.

Now, according to this text (Warning: about D6! Has anything changed here in D7? - look at Case 4 there) it should be translatable.

Is this a bug in og_ui? Why can't I translate this title string?