FFMPeg uses the ISO 639-2 codes.
-metadata:s:1 language=eng
NOTE: Sets metadata language to eng on the stream id 1 (which is, in typical cases, the first audio stream).
-metadata:s:s:0 language=eng
NOTE: Sets the metadata language to eng on the first subtitle stream.
-map 0:v \ -map 0:1 -metadata:s:a:0 language=zxx \ -map 0:2 -metadata:s:a:1 language=eng \ -map 0:3 -metadata:s:a:2 language=fra \ -map 0:4 -metadata:s:a:3 language=kin
NOTE: The standard includes some codes for special situations:
ffmpeg -i in.mkv -map 0:a -metadata:s:a:0 language="und" -metadata:s:a:0 title="Just Music" out.mkv
NOTE: und stands for Undetermined.
The standard includes some codes for special situations: