Resolves the GCP speaking rate for a listen language.
Precedence: per-language env override → global env → built-in calibration → 1.0.
Listen language code (e.g. fr, zh).
fr
zh
Optional env snapshot (defaults to process.env).
process.env
Speaking rate in [0.25, 2.0].
[0.25, 2.0]
Resolves the GCP speaking rate for a listen language.
Precedence: per-language env override → global env → built-in calibration → 1.0.