player/loadfile: match starting part of the language code

This allows to match fr-CA directly or match fr- to allow fr-CA and
fr-FR.

DOCS/man/options.rst

@@ -16,22 +16,23 @@ Track Selection
 
  - ``mpv dvd://1 --alang=hu,en`` chooses the Hungarian language track
  on a DVD and falls back on English if Hungarian is not available.
- - ``mpv --alang=jpn example.mkv`` plays a Matroska file with Japanese
+ - ``mpv --alang=ja-JP,jpn example.mkv`` plays a Matroska file with Japanese
  audio.
 
 ``--slang=<languagecode[,languagecode,...]>``
- Specify a priority list of subtitle languages to use. Different container
- formats employ different language codes. DVDs use ISO 639-1 two letter
- language codes, Matroska uses ISO 639-2 three letter language codes while
- OGM uses a free-form identifier. See also ``--sid``.
+ Equivalent to ``--alang``, for subtitle tracks.
 
  This is a string list option. See `List Options`_ for details.
 
  .. admonition:: Examples
 
  - ``mpv dvd://1 --slang=hu,en`` chooses the Hungarian subtitle track on
  a DVD and falls back on English if Hungarian is not available.
- - ``mpv --slang=jpn example.mkv`` plays a Matroska file with Japanese
+ - ``mpv --slang=ja-JP,jpn example.mkv`` plays a Matroska file with Japanese
+ subtitles.
+ - ``mpv --slang=pt-BR,pt-PT example.mkv`` plays a Matroska file with Brazilian
+ Portuguese subtitles if available, and otherwise any Portuguese subtitles.
+ - ``mpv --slang=pt example.mkv`` plays a Matroska file with any Portuguese
  subtitles.
 
 ``--vlang=<...>``

player/loadfile.c

@@ -456,7 +456,7 @@ static int match_lang(char **langs, const char *lang)
  if (!lang)
  return 0;
  for (int idx = 0; langs && langs[idx]; idx++) {
- if (lang && strcasecmp(langs[idx], lang) == 0)
+ if (lang && strncasecmp(langs[idx], lang, strlen(langs[idx])) == 0)
  return INT_MAX - idx;
  }
  return 0;