options: default slang to auto

DOCS/interface-changes.rst

@@ -79,6 +79,7 @@ Interface changes
     - add `--gamut-mapping-mode=perceptual|relative|saturation|absolute|linear`
     - add `--corner-rounding` option
     - change `--subs-with-matching-audio` default from `yes` to `no`
+    - change `--slang` default from blank to `auto`
  --- mpv 0.35.0 ---
     - add the `--vo=gpu-next` video output driver, as well as the options
       `--allow-delayed-peak-detect`, `--builtin-scalers`,

DOCS/man/options.rst

@@ -24,7 +24,7 @@ Track Selection
           audio.
 
 ``--slang=<languagecode[,languagecode,...]>``
-    Equivalent to ``--alang``, for subtitle tracks.
+    Equivalent to ``--alang``, for subtitle tracks (default: auto).
 
     This is a string list option. See `List Options`_ for details.
 

options/options.c

@@ -1030,6 +1030,11 @@ static const struct MPOpts mp_default_opts = {
                    { [STREAM_AUDIO] = -2,
                      [STREAM_VIDEO] = -2,
                      [STREAM_SUB] = -2, }, },
+    .stream_lang = {
+        NULL, // video
+        NULL, // audio
+        (char**)(const char*[]) {"auto", NULL} // subtitles
+    },
     .stream_auto_sel = true,
     .subs_with_matching_audio = false,
     .audio_display = 1,