player: add --subs-match-os-language option

This is the replacement for the previous auto option for slang. It
behaves similar however it never overrides slang if that is set and will
instead try to pick the subtitle that matches the user's language if
appropriately flagged by the file.

DOCS/interface-changes.rst

@@ -45,6 +45,7 @@ Interface changes
     - add `--audio-file-exts`, `--cover-art-auto-exts`, and `--sub-auto-exts`
     - change `slang` default back to NULL
     - remove special handling of the `auto` value from `--alang/slang/vlang` options
+    - add `--subs-match-os-language` as a replacement for `--slang=auto`
  --- mpv 0.36.0 ---
     - add `--target-contrast`
     - Target luminance value is now also applied when ICC profile is used.

DOCS/man/options.rst

@@ -139,6 +139,12 @@ Track Selection
     When autoselecting a subtitle track, select a full/non-forced one even if the selected
     audio stream matches your preferred subtitle language (default: yes).
 
+``--subs-match-os-language=<yes|no>``
+    When autoselecting a subtitle track, select the track that matches the language of your OS
+    if the audio stream is in a different language if suitable (default track or a forced track
+    under the right conditions). Note that if ``-slang`` is set, this will be completely ignored
+    (default: yes).
+
 ``--subs-fallback=<yes|default|no>``
     When autoselecting a subtitle track, if no tracks match your preferred languages,
     select a full track even if it doesn't match your preferred subtitle language (default: default).

options/options.c

@@ -522,6 +522,7 @@ static const m_option_t mp_opts[] = {
     {"vlang", OPT_STRINGLIST(stream_lang[STREAM_VIDEO])},
     {"track-auto-selection", OPT_BOOL(stream_auto_sel)},
     {"subs-with-matching-audio", OPT_BOOL(subs_with_matching_audio)},
+    {"subs-match-os-language", OPT_BOOL(subs_match_os_language)},
     {"subs-fallback", OPT_CHOICE(subs_fallback, {"no", 0}, {"default", 1}, {"yes", 2})},
     {"subs-fallback-forced", OPT_BOOL(subs_fallback_forced)},
 
@@ -1044,6 +1045,7 @@ static const struct MPOpts mp_default_opts = {
                      [STREAM_SUB] = -2, }, },
     .stream_auto_sel = true,
     .subs_with_matching_audio = true,
+    .subs_match_os_language = true,
     .subs_fallback = 1,
     .subs_fallback_forced = true,
     .audio_display = 1,

options/options.h

@@ -268,6 +268,7 @@ typedef struct MPOpts {
     char **stream_lang[STREAM_TYPE_COUNT];
     bool stream_auto_sel;
     bool subs_with_matching_audio;
+    bool subs_match_os_language;
     int subs_fallback;
     bool subs_fallback_forced;
     int audio_display;

player/loadfile.c

@@ -472,10 +472,11 @@ static int match_lang(char **langs, const char *lang)
  * 1) track is external (no_default cancels this)
  * 1b) track was passed explicitly (is not an auto-loaded subtitle)
  * 1c) track matches the program ID of the video
- * 2) earlier match in lang list
+ * 2) earlier match in lang list but not if we're using os_langs
  * 3a) track is marked forced and we're preferring forced tracks
  * 3b) track is marked non-forced and we're preferring non-forced tracks
  * 3c) track is marked default
+ * 3d) match in lang list with os_langs
  * 4) attached picture, HLS bitrate
  * 5) lower track number
  * If select_fallback is not set, 5) is only used to determine whether a
@@ -485,7 +486,7 @@ static int match_lang(char **langs, const char *lang)
  */
 // Return whether t1 is preferred over t2
 static bool compare_track(struct track *t1, struct track *t2, char **langs,
-                          struct MPOpts *opts, int preferred_program)
+                          bool os_langs, struct MPOpts *opts, int preferred_program)
 {
     if (!opts->autoload_files && t1->is_external != t2->is_external)
         return !t1->is_external;
@@ -505,10 +506,12 @@ static bool compare_track(struct track *t1, struct track *t2, char **langs,
             return t1->program_id == preferred_program;
     }
     int l1 = match_lang(langs, t1->lang), l2 = match_lang(langs, t2->lang);
-    if (l1 != l2)
+    if (!os_langs && l1 != l2)
         return l1 > l2;
     if (t1->default_track != t2->default_track)
         return t1->default_track;
+    if (os_langs && l1 != l2)
+        return l1 > l2;
     if (t1->attached_picture != t2->attached_picture)
         return !t1->attached_picture;
     if (t1->stream && t2->stream && opts->hls_bitrate >= 0 &&
@@ -546,19 +549,19 @@ static bool append_lang(size_t *nb, char ***out, char *in)
     return true;
 }
 
-static bool add_auto_langs(size_t *nb, char ***out)
+static char **add_os_langs(void)
 {
-    bool ret = false;
+    size_t nb = 0;
+    char **out = NULL;
     char **autos = mp_get_user_langs();
     for (int i = 0; autos && autos[i]; i++) {
-        if (!append_lang(nb, out, autos[i]))
+        if (!append_lang(&nb, &out, autos[i]))
             goto cleanup;
     }
-    ret = true;
 
 cleanup:
     talloc_free(autos);
-    return ret;
+    return out;
 }
 
 static char **process_langs(char **in)
@@ -611,6 +614,13 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
     if (tid == -2)
         return NULL;
     char **langs = process_langs(opts->stream_lang[type]);
+    bool os_langs = false;
+    // Try to add OS languages if enabled by the user and we don't already have a lang from slang.
+    if (type == STREAM_SUB && (!langs || !strcmp(langs[0], "")) && opts->subs_match_os_language) {
+        talloc_free(langs);
+        langs = add_os_langs();
+        os_langs = true;
+    }
     const char *audio_lang = get_audio_lang(mpctx);
     bool sub = type == STREAM_SUB;
     bool fallback_forced = sub && opts->subs_fallback_forced;
@@ -633,12 +643,12 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
             continue;
         if (duplicate_track(mpctx, order, type, track))
             continue;
-        if (!pick || compare_track(track, pick, langs, mpctx->opts, preferred_program))
+        if (!pick || compare_track(track, pick, langs, os_langs, mpctx->opts, preferred_program))
             pick = track;
 
         // We only try to autoselect forced tracks if they match the audio language and are subs
         if (fallback_forced && track->forced_track && mp_match_lang_single(audio_lang, track->lang) &&
-            (!forced_pick || compare_track(track, forced_pick, langs, mpctx->opts, preferred_program)))
+            (!forced_pick || compare_track(track, forced_pick, langs, os_langs, mpctx->opts, preferred_program)))
             forced_pick = track;
     }
 
@@ -652,7 +662,7 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
         sub_fallback = (pick->is_external && !pick->no_default) || opts->subs_fallback == 2 ||
                         (opts->subs_fallback == 1 && pick->default_track);
     }
-    if (pick && !forced_pick && sub && !match_lang(langs, pick->lang) &&
+    if (pick && !forced_pick && sub && (!match_lang(langs, pick->lang) || os_langs) &&
         ((!opts->subs_with_matching_audio && audio_matches) || !sub_fallback))
         pick = NULL;