--slang region doesn't work

[Piber0]

Important Information

Provide following Information:

• mpv version 0.38.0
• Windows 11 23H2
• Source of the mpv binary: https://sourceforge.net/projects/mpv-player-windows/files/release/mpv-0.38.0-x86_64-v3.7z/download
• If known which version of mpv introduced the problem
• GPU model, driver and version
• Possible screenshot or video of visual glitches

If you're not using git master or the latest release, update.
Releases are listed here: https://github.com/mpv-player/mpv/releases

Reproduction steps

mpv --no-config --slang=fr-FR output.mkv

Expected behavior

According to the documentation, the slang option supports the region of a language, so this command should select the subtitle French from France (fr-FR) instead of French from Canada (fr-CA).

Actual behavior

Log file

Z:\>mpv --no-config --slang=fr-FR output.mkv
 (+) Video --vid=1 (*) (h264 1920x1080 23.976fps)
 (+) Audio --aid=1 --alang=en (*) (eac3 6ch 48000Hz)
 (+) Subs  --sid=1 --slang=fr-CA (subrip)
     Subs  --sid=2 --slang=fr-FR (subrip)
AO: [wasapi] 48000Hz stereo 2ch float
VO: [gpu] 1920x1080 yuv420p
AV: 00:00:02 / 00:43:29 (0%) A-V: -0.000 Cache: 84s/150MB

Sample files

The specific region of a subtitle can be created in MKVToolNix GUI (and mkvpropedit propably too).

[kasper93]

Fixed by #13917