Function filterHlsMainPlaylistByLanguage

Filters an HLS main playlist by preferred language using BCP 47 relatedness matching.

Strategy:

1. Collect all unique languages from audio alternative renditions.
2. Find the best matching language using relatedness scoring.
3. If a match is found, keep only renditions at the same relatedness level (or better), plus renditions without a language tag. Filter variants whose audio group has no remaining matching renditions.
4. If no match is found or filtering would remove all variants, return unchanged.