Allow to select unknown language with --slang

Expected behavior of the wanted feature

I have an MKV with a subtitle track that has no language tag. The OSD displays it as “1/1 [unknown] Subtitles”.

It would be nice if --slang would accept a special keyword, maybe unknown, to select the first subtitle track with unknown language.

Note that --sid will not work in the config file because it overwrites --slang.

Alternative behavior of the wanted feature

A new option for selecting a fallback sid would also work for me, but the special keyword in --slang would be more flexible.

Log file

output.txt

The subtitle is detected as this:

[…]
[   0.062][i][cplayer]      Subs  --sid=1 'Subtitles' (ass)
[…]

Workaround

In case someone else has the same problem: If you put --use-filedir-conf in the config file, mpv will try to load a mpv.conf from the same directory as the file played. So you could put --sid=1 in there. See the section “File-specific Configuration Files” in the man-page.

Start by tracing how the --slang option selects subtitle tracks and how --sid interacts with configuration files; the issue mentions the man-page section “File-specific Configuration Files” for related behavior. Done means --slang accepts a special value such as unknown and selects the first subtitle track without a language tag, while preserving existing --sid behavior.