added translation for navigation

lms/templates/courseware/course_navigation.html

 ## mako
+<%! from django.utils.translation import ugettext as _ %>
 <%page args="active_page=None" />
 
 <%
@@ -20,7 +21,7 @@ def url_class(is_active):
       % for tab in get_course_tabs(user, course, active_page):
           <li>
              <a href="${tab.link | h}" class="${url_class(tab.is_active)}">
-                 ${tab.name | h}
+                 ${_(tab.name | h)}
                  % if tab.is_active == True:
                   <span class="sr">, current location</span>
                  %endif