demux/subtitle: improved language detection
Highlights:
- Now makes use of the language property in SSA/ASS files when available, as supported since libass v0.10.0. (This is currently a simple 2-char ISO-639-1 code).
- Fixes ugly filename based language name extraction failures involving incorrectly capturing portion of directory names.
- Adds handling for common alternate
PATH/Subs/x_LANG.ext
type pattern for filename language name extraction.