I have the following code in Transmageddon for trying to set some tags. I have noticed that the TAG_APPLICATION_NAME tag gets set in Quicktime container, but the TAG_LANGUAGE_CODE doesn't seem to be set in any format I tried (Vorbis, WebM and Quicktime). 

   def OnEncodebinElementAdd(self, encodebin, element):
       factory=element.get_factory()
       if factory != None:
           # set multipass cache file on video encoder element
           if (self.multipass != 0) and (self.passcounter == int(0)):
               if Gst.ElementFactory.list_is_type(factory, 2814749767106562): # this is the factory code for Video encoders
                   element.set_property("multipass-cache-file", self.cachefile)
           
           # Set Transmageddon as Application name using Tagsetter interface
           tagyes = factory.has_interface("GstTagSetter")
           if tagyes ==True:
               taglist=Gst.TagList.new_empty()
               taglist.add_value(Gst.TagMergeMode.APPEND, Gst.TAG_APPLICATION_NAME, "Transmageddon transcoder")
               element.merge_tags(taglist, Gst.TagMergeMode.REPLACE)
           if Gst.ElementFactory.list_is_type(factory, 1125899906842626): # Audio Encoders factory code
                   print("got audio encoder")
                   taglistBANG=Gst.TagList.new_empty()
                   taglistBANG.add_value(Gst.TagMergeMode.APPEND, Gst.TAG_LANGUAGE_CODE, self.audiodata[0]['language'])  # FIXME: Currently only doing 1 stream
                   print(self.audiodata[0]['language'])
                   longname=factory.get_metadata('long-name')
                   print(longname)
                   taglistBANG.add_value(Gst.TagMergeMode.APPEND, Gst.TAG_ENCODER, longname)
                   element.merge_tags(taglistBANG, Gst.TagMergeMode.REPLACE)