mpv ignoring 'slang' option set on 'mpv.conf' in favor of external subs

mpv Information

mpv v0.38.0-476-gd2bd77ad Copyright © 2000-2024 mpv/MPlayer/mplayer2 projects
 built on Jun  9 2024 00:05:17
libplacebo version: v7.349.0 (v7.349.0-rc1-2-gbc9de9c-dirty)
FFmpeg version: N-115628-g658439934
FFmpeg library versions:
   libavutil       59.21.100
   libavcodec      61.7.100
   libavformat     61.3.104
   libswscale      8.2.100
   libavfilter     10.2.102
   libswresample   5.2.100

Other Information

• Windows version: 10.0.19045.4529
• GPU model, driver and version: RTX 4060, GeForce game ready 555.99
• Source of mpv: sourceforge

Reproduction Steps

1. Download video with embedded subtitles (in my case, English).
2. Download separate substitles (in my case, Japanese) and rename them so their name matches that of the video file.
3. Put the subs in a separate directory and set a path for that directory on mpv.conf with sub-file-paths
4. Set slang in mpv so that the external subs' language is last on the priority list (in my case, slang=eng,en,enUS,jpn,jp.
5. Open video and watch it as mpv ignores the priority list set in slang (choosing the external subs by default).

Expected Behavior

In my case, I was expecting mpv to respect the priority list in slang and choose the (embedded) English subs by default.

Actual Behavior

Mpv instead seemed to ignore the list in slang and set the (external) Japanese subs by default.

Log File

output.txt

Sample Files

No response

I carefully read all instruction and confirm that I did the following:

• I tested with the latest mpv version to validate that the issue is not already fixed.
• I provided all required information including system and mpv version.
• I produced the log file with the exact same set of files, parameters, and conditions used in "Reproduction Steps", with the addition of --log-file=output.txt.
• I produced the log file while the behaviors described in "Actual Behavior" were actively observed.
• I attached the full, untruncated log file.
• I attached the backtrace in the case of a crash.

Start by reviewing output.txt and the reproduction involving slang and sub-file-paths, then trace how mpv selects embedded versus external subtitles. Done means the configured language priority selects the embedded English subtitles instead of defaulting to the external Japanese subtitles.