Description:
All attempts to visualize subtitles ripped into the mkv file fail:
* mpv bla.mkv
* mpv --sid=auto bla.mkv
* mpv --sid=3 bla.mkv

Also trying to visualize extracted subtitles (.idx & .sub files) failes:
* mkvextract

All attempts always end in no subtitles being loaded and the following to lines seem to hold the answer:

[sub/ass] Could not open libavcodec subtitle converter
[sub] Could not find subtitle decoder for format 'dvd_subtitle'.

See [1] below for full output.

Additional info:
* package version:
mpv 0.25.0 (C) 2000-2017 mpv/MPlayer/mplayer2 projects
built on Mon May 8 20:03:32 CEST 2017
ffmpeg library versions:
libavutil 55.58.100
libavcodec 57.89.100
libavformat 57.71.100
libswscale 4.6.100
libavfilter 6.82.100
libswresample 2.7.100
ffmpeg version: 3.3.1

* config and/or log files etc.
see [1]

Steps to reproduce:
* Buy Litter-Kwitter ;)
* Rip packaged DVD with Subtitles using Handbrake into x265 video, mkv file format, selecting all subtitles
* ... or download the video file in question (small):
http://mir.cr/1NH7JWJK
* Play video file trying to activate subtitles

[1]
mpv VTS_01_1.mkv --sid=15
Resuming playback. This behavior can be disabled with --no-resume-playback.
Loading config '/home/me/.config/mpv/watch_later/FC120F4ECC02C60C9AE1856560A4CF66'
Playing: VTS_01_1.mkv
[ffmpeg] IMGUTILS: Picture size 0x0 is invalid
[ffmpeg/demuxer] vobsub: Failed to open codec in avformat_find_stream_info
(+) Video --vid=1 (*) (hevc 720x480 29.970fps)
(+) Audio --aid=1 (*) 'Stereo' (aac 2ch 48000Hz)
Subs --sid=1 (dvd_subtitle)
Subs --sid=2 (dvd_subtitle)
Subs --sid=3 (dvd_subtitle)
Subs --sid=4 (dvd_subtitle)
Subs --sid=5 (dvd_subtitle)
Subs --sid=6 (dvd_subtitle)
Subs --sid=7 (dvd_subtitle)
Subs --sid=8 (dvd_subtitle)
Subs --sid=9 (dvd_subtitle)
Subs --sid=10 (dvd_subtitle)
Subs --sid=11 (dvd_subtitle)
Subs --sid=12 (dvd_subtitle)
Subs --sid=13 (dvd_subtitle)
Subs --sid=14 (dvd_subtitle)
(+) Subs --sid=15 --slang=en (*) 'VTS_01_1.idx' (dvd_subtitle) (external)
File tags:
Title: VTS_01_1
[ffmpeg] IMGUTILS: Picture size 0x0 is invalid
[sub/lavc] Could not open libavcodec subtitle decoder
[ffmpeg] IMGUTILS: Picture size 0x0 is invalid
[sub/ass] Could not open libavcodec subtitle converter
[sub] Could not find subtitle decoder for format 'dvd_subtitle'.
AO: [pulse] 48000Hz stereo 2ch float
Using hardware decoding (vaapi).
VO: [opengl] 720x480 => 720x540 vaapi[nv12]
AV: 00:00:52 / 00:04:24 (19%) A-V: 0.000
Saving state.