Error messages named the resolved language for a user who only ever defaulted to orig. Audio tracks now carry is_original, resolved through Tracks.by_language so it agrees with the downloader.
by_language now applies RFC 4647 Lookup in exact mode, preferring the string-equal tag so --exact-lang separates base/regional pairs (ar/ar-EG, en/en-US) instead of collapsing them, and dedupes by id to prevent the duplicate-track-id crash. Adds a language-selection regression test.
Missing requested subtitle and audio languages now warn and continue when --best-available is set instead of hard-exiting. Without the flag, missing languages still produce an error and exit, matching the prior strict behavior. Audio missing-lang detection is now symmetric with subtitles.
- add find_missing_langs helper in core/utilities for reuse between s_lang and a_lang paths (skips all/best/orig sentinels)
- refactor dl.py s_lang/a_lang checks to share the helper
- add tests/lang_selection covering match primitives, helper output, and tricky langcodes corners (zh-Hans/zh-Hant/zh-CN/zh-TW/zh-HK, cmn/yue, fil/tl/tgl)
- clean up unused-var ruff F841 in tests/remote/unit/
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