I wish to translate the same mp4 video into multiple language sub tracks. Can you offer the specification of multiple languages in the --target_langauge parameter? Then, the process of transcribing audio wouldn't have to be repeated multiple times (you could also offer an option to not recreate transcribed audio if the resulting transcripted SRT file is already present, to save time - if this option is not specified then the package could ask the user to recreate the file or use the existing one).
Then, possibly, you could have the option to run the translations in subprocesses since they do not depend on one another, and only on prerequisites already computed, this could be disabled on smaller systems without the computing capacity to accommodate this.
PS. I have a particular process, when I have translated SRT files in hand (original English, Simplified Chinese, Korean in my case) I use the following command to quickly have the subs contained within my single mp4 video file, and I use a tool like VLC to select a sub track (by default no subs are displayed) :
ffmpeg -i Lesson3.mp4 -i Lesson3.srt -i Lesson3_SimplifiedChinese.srt -i Lesson3_Korean.srt -map 0:v -map 0:a -map 1:s -map 2:s -map 3:s -c:v copy -c:a copy -c:s mov_text Lesson3_Subtitled.mp4
I am not combining dual languages in my SRT files, although I can see some users may need this, I only need the base language SRT files as input to my process.