ffmpeg -i input.mp4 -i subtitle.en.srt -i subtitle.chi.srt -map 0 -map 1 -map 2 -c copy -c:s mov_text -metadata:s:s:0 language=eng -metadata:s:s:1 language=chi output_eng_chi.mp4
NOTE: This adds two
This assumes that the input file already has a subtitle.
ffmpeg -i input_english.mp4 -i subtitle.chi.srt -map 0 -map 1 -c copy -c:s mov_text -metadata:s:s:1 language=chi output_chi.mp4
NOTE:
Without -map, ffmpeg will select only one subtitle stream among all inputs.