> Before this step, it has the zh file already, right? Then this step overwrites that file? Why does this step change the default to uk? (Same question asked above)

For the newly created video module it uses a first translation from uploaded translations sorted by language code(http://loc.gov/standards/iso639-2/ISO-639-2_utf-8.txt according to http://en.wikipedia.org/wiki/ISO_639-1) as primary. 
After merging `sub` and `transcripts` fields, we'll add possibility to choose primary translation, but it's a part of another PR.
[Code is here](https://github.com/edx/edx-platform/blob/alex/multiple_transcripts_editor/common/lib/xmodule/xmodule/video_module/video_module.py#L123)