The spoken language is only knowable when the transcription model reports it,
and only whisper.cpp does: the hosted endpoints accept auto but never say what
they heard. transcribe_detected() asks for detection exactly where it can be
answered, the local server in auto mode, by requesting verbose_json with
no_language_probabilities switched back on for that one request (the server
runs with -nlp, which keeps the sweep off everything else).
