Ability to use English subtitle file as text for translation field.

I haven't found a way to use English subs from the video file as translation text for the corresponding target language subs. I know this is not the best practice when it comes to language learning, but I think it could be useful in some cases. 
The user should be asked to specify the sid of the native language subtitle file when a `translation` variable is used in the immersive-targets.conf