i18n: Make translation works for video editor tabs

Now the display_name & help strings in video editor tabs can be extracted and translated.

common/lib/xmodule/xmodule/video_module/video_module.py

@@ -44,6 +44,7 @@ def get_ext(filename):
 
 
 log = logging.getLogger(__name__)
+_ = lambda text: text
 
 
 class VideoModule(VideoFields, VideoStudentViewHandlers, XModule):
@@ -177,12 +178,12 @@ class VideoDescriptor(VideoFields, VideoStudioViewHandlers, TabsEditingDescripto
 
     tabs = [
         {
-            'name': "Basic",
+            'name': _("Basic"),
             'template': "video/transcripts.html",
             'current': True
         },
         {
-            'name': "Advanced",
+            'name': _("Advanced"),
             'template': "tabs/metadata-edit-tab.html"
         }
     ]
@@ -358,7 +359,7 @@ class VideoDescriptor(VideoFields, VideoStudioViewHandlers, TabsEditingDescripto
         _ = self.runtime.service(self, "i18n").ugettext
         video_url.update({
             'help': _('The URL for your video. This can be a YouTube URL or a link to an .mp4, .ogg, or .webm video file hosted elsewhere on the Internet.'),
-            'display_name': 'Default Video URL',
+            'display_name': _('Default Video URL'),
             'field_name': 'video_url',
             'type': 'VideoList',
             'default_value': [get_youtube_link(youtube_id_1_0['default_value'])]

common/lib/xmodule/xmodule/x_module.py

@@ -788,11 +788,14 @@ class XModuleDescriptor(XModuleMixin, HTMLSnippet, ResourceTemplates, XBlock):
             if field.scope != Scope.settings or field in self.non_editable_metadata_fields:
                 continue
 
+            def get_text(value):
+                return self.runtime.service(self, "i18n").ugettext(value) if value is not None else None
+
             # gets the 'default_value' and 'explicitly_set' attrs
             metadata_fields[field.name] = self.runtime.get_field_provenance(self, field)
             metadata_fields[field.name]['field_name'] = field.name
-            metadata_fields[field.name]['display_name'] = field.display_name
-            metadata_fields[field.name]['help'] = field.help
+            metadata_fields[field.name]['display_name'] = get_text(field.display_name)
+            metadata_fields[field.name]['help'] = get_text(field.help)
             metadata_fields[field.name]['value'] = field.read_json(self)
 
             # We support the following editors: