Ubuntu – VLC : Error, Unable to save subtitles

It looks like it stays in memory to me. A similar plugin that just downloads the subtitles file from opensubtitles is probably going to be your answer by using some of that code and modifying around line:

--vlc.msg.dbg("[Subtitles] File found in the archive: " .. srturl .. extension)  

The one line edit below is BAD, but gets it not broken for unix-like OS-es only. For the script to be cross platform it needs OS detection logic:

if(item ~= nil) then
  local name = item:uri()
  vlc.msg.info("NAME: "..name)
  name = vlc.strings.decode_uri(string.gsub(name, "file:///", ""))      
  name = "/"..name
  vlc.msg.info(""..name.."."..language.."."..extension)
  vlc.msg.info("[Subtitle-download] saving subtitle to: "..name.."."..language.."."..extension)
  local fsout = assert(io.open(name.."."..language.."."..extension, "w"))
  fsout:write(dataBuffer)
  assert(fsout:close())
end

If someone running Windows VLC is interested in testing a patch that gets saving working on both Linux and Windows, check this pull request.

Evidently someone has tacked on some enhancements including the patch here.

This version here works in Linux (as well as in Windows) to save subtitles.

To provide an actual mpv only answer, you can specify the mechanism by which subtitle tracks are loaded in your mpv.conf file located at .config/mpv.conf as explained here in the mpv manual. In your case you would add sub-auto=all.