Update the subtitle language when we perform an action (#1752)

The issue was that we weren't calling it if the user performed an
action without any edits to the subtitles.  In that case the code path
goes through Workflow.perform_action() and that method had a bug.
Merge branch 'staging' into gh-1752
Fixed unittest (#1752)
Merge pull request #1889 from pculture/gh-1752

Gh 1752