I18n command tweaks

This depends on #1110 and begins to address #1124
See 7e8bace and 23945c7; true diff here: https://github.com/bcipolli/ka-lite/compare/dubbed_video_support...i18n_command_tweaks?expand=1


Just a few tweaks to shorten up code, avoid blocking progress on a single unknown language, and documentation.