====== ffmpeg - Only keep English ======

<code bash>
ffmpeg -i "input.mkv" -c copy -map 0:v -map 0:a:m:language:eng? -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"
ffmpeg -i "input.mkv" -map 0:v -map 0:a:m:language:eng? -c copy -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"

or

ffmpeg -i "input.mkv" -map 0:0 -map 0:a:m:language:eng? -c copy -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"
ffmpeg -i "input.mkv" -c copy -map 0:v -map 0:a:m:language:eng? -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"
</code>

<WRAP info>
**NOTE:**  

  * **-c:s copy**:  Using stream copy to remux (no encoding).
  * **-map 0:v**:  Keep the existing Video.
    * **-map 0:0**:  Keep the existing map 0:0.  (Assume this is the Video).
  * **-map 0:a:m:language:eng?**:  Keep only English Audio. (Optional only if Audio exists).
  * **-map 0:s:m:language:eng?**:  Keep only English Subtitles. (Optional only if Subtitles exist).
  * **-map 0:d?**:  Keep Data. (Optional only if Data exists).
  * **-map 0:t?**:  Keep Attachments. (Optional only if Attachments exists).

</WRAP>

----

<code bash>

# QUICK.
ffmpeg -i "input.mkv" -c copy -map 0:v -map 0:a:m:language:eng? -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "out1.mkv"

# SLOW, BUT SLIGHTLY SMALLER.
ffmpeg -i "input.mkv" -c:v copy -map 0:v -map 0:a:m:language:eng? -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"

# VERY SLOW, BUT SMALLER.
ffmpeg -i "input.mkv" -map 0:v -c:a copy -map 0:a:m:language:eng? -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"

# EXTREMELY SLOW, BUT SMALLEST.
ffmpeg -i "input.mkv" -map 0:v -map 0:a:m:language:eng? -c:s copy -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"
ffmpeg -i "input.mkv" -map 0:v -map 0:a:m:language:eng? -map 0:s:m:language:eng? -map 0:d? -map 0:t? "output.mkv"

</code>

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===== References =====

https://trac.ffmpeg.org/wiki/Map