I found that the HEAD of seahorse does not show menu entries with my locale (Japanese).
The keymanager in seahorse has used GtkActionEntry to build menu ui_entries, howerver, it seems to be lost gtk_action_group_set_translation_domain() to them:

seahorse-key-manager.c:1022
    /* General normal actions */
    actions = gtk_action_group_new ("main");
    gtk_action_group_add_actions (actions, ui_entries,
                                  G_N_ELEMENTS (ui_entries), swidget);
    seahorse_widget_add_actions (swidget, actions);
should be 
    /* General normal actions */
    actions = gtk_action_group_new ("main");
    gtk_action_group_set_translation_domain (actions, NULL);
    gtk_action_group_add_actions (actions, ui_entries,
                                  G_N_ELEMENTS (ui_entries), swidget);
    seahorse_widget_add_actions (swidget, actions);

Here is my workaround as one patch.

I'm not sure what version of seahorse has this issue from, maybe v0.8.x branch  has also.