Language Code for Subtitle Tracks (in the description field)

## Subtitle tracks

To add subtitles to your video, upload a VTT file to the file field, or
link to a VTT file when using a Link field. Enter the language name or
two-letter code in the description field. Use the English or local language
name.

I tried to upload VTT files for subtitle track and it works great. Thanks for this fantastic work.
But I found a small bug regarding the two-letter language code:
When I enter the two-letter ode in the description field, only the "zh" (Chinese) doesn't work. There is nothing appeared in the subtitle menu.
All the other languages, like "en", "jo", "fr", "nl"... just work fine ( I almost tried all the srclang codes https://github.com/videojs/video.js/blob/stable/docs/guides/tracks.md).

Does anyone know how to solve this problem.