It also reads AVI
Here is my primitive batch file for Windows, which is probably very easy to convert to bash script
for %%f in (*.avi) do (
echo %%~nf
"C:\Program Files\MKVToolNix\mkvmerge" -o "%%~nf.mkv" --language 0:ger --language 1:jpn "%%~nf.avi"
)
It tags the video stream 0 with a language as well (I use this for anime to indicate the language of “hard” subs. This won’t appear in Plex though, except in the mediainfo.)