get_lyrics_url_raw/get_lyrics_url/get_lyrics take an optional ISO 639-1
language, appended to the signed param list only when Some (the existing
sign_request sort covers it). None keeps the original-only default flow.
When a translation was requested but the document does not embed one,
the envelope's translation_requested.lyrics_url is fetched as a fallback
translation source (spec section B.3); every gap degrades to
original-only. lyrics_probe gains --lang=<iso639-1> for parity.
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vicrodh
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