Hm... You no need `Youtube_with_download`, use the following code to change fields:
```lettuce
Given the course has a Video component in Youtube mode:
  | transcripts           | download_track |
  | {"zh": "OEoXaMPEzfM"} | true          |
```
and add [here](https://github.com/edx/edx-platform/blob/alex/fix_download_html5_subs/lms/djangoapps/courseware/features/video.py#L137) something like:
```python
 if 'download_track' in kwargs['metadata']:
        kwargs['metadata']['download_track'] = json.loads(kwargs['metadata']['download_track'])
```
, but I'd prefer to make it more general like [here](https://github.com/edx/edx-platform/blob/6cdeac2b6fb46e732ba1f68f49e6f9f2d0d553f5/common/lib/xmodule/xmodule/video_module/video_module.py#L236):
```python
conversions = {
     'speed': json.loads,
     'saved_video_position': lambda v: RelativeTime.isotime_to_timedelta(v),
}
if key in conversions:
      value = conversions[key](data[key])
else:
      value = data[key]
```