How to copy or convert multiple audio and subtitle streams:
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Command:
ffmpeg -i "old.mkv" -map 0:a? -map 0:s? -map 0:v -c:v copy -c:a copy -c:s copy "new.mkv"

The -map 0:a? is to get all audio streams listed.
The -map 0:s? is to get all audio subtitles listed.
The -map 0:v? is to get all video streams listed.

"copy" means to not have any conversion for that stream, and input untoched data into the new file. You can replace this with any conversion you want.
-c:v is for video
-c:a is for audio
-c:s is for subtitle

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Source: https://superuser.com/questions/1273764/using-ffmpeg-on-how-do-i-copy-the-video-and-multiple-subtitle-streams-in-an-mkv
Date: June 3rd, 2021