https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/subtitle_options

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Not every subtitle codec can be used for every video container format!

[subtitle codec] parameter examples:
for MKV containers: copy, ass, srt, ssa
for MP4 containers: copy, mov_text
for MOV containers: copy, mov_text

Note: Many Philips Blu-Ray Players, Samsung Smart TV's and other 
standalone players can only read "SRT" subtitle streams in "MKV" files.

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FFmpeg command-line examples to importing subtitle files
Import a subtitle file (copy video/audio streams, without re-encoding)

ffmpeg -i input.mp4 -sub_charenc CP1252 -i subtitle.srt -map 0:v -map 0:a -c copy -map 1 -c:s:0 mov_text -metadata:s:s:0 language=ger output.mp4

ffmpeg -i input.mkv -sub_charenc CP1252 -i subtitle.srt -map 0:v -map 0:a -c copy -map 1 -c:s:0 srt -metadata:s:s:0 language=fre output.mkv

or

ffmpeg -i input.mp4 -sub_charenc CP1252 -i subtitle.srt -vcodec copy -acodec copy -scodec mov_text -metadata:s:s:0 language=ger output.mp4

ffmpeg -i input.mkv -sub_charenc CP1252 -i subtitle.srt -vcodec copy -acodec copy -scodec srt -metadata:s:s:0 language=fre output.mkv

Example for 2 separate .srt subtitle files:

ffmpeg -i input.mp4       -i subtitle_eng.srt -i subtitle_fre.srt \
       -map 0:v -map 0:a  -map 1              -map 2              \
       -c:v copy -c:a copy -c:s copy                              \
       -metadata:s:s:0 language=eng -metadata:s:s:1 language=fre  \
  output.mkv

Import subtitle and re-encode video/audio streams

ffmpeg -i input.mov -sub_charenc WINDOWS-1252 -i subtitle.srt -vcodec h264 -acodec mp3 -scodec mov_text -metadata:s:s:0 language=spa output2.mp4

ffmpeg -i input.avi -sub_charenc ISO-8859-1 -i subtitle.srt -vcodec h264 -acodec ac3 -scodec srt -metadata:s:s:0 language=ita output2.mkv

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