Auto language detection returns English only for mixed English/Spanish audio

TL;DR

Batch transcription of a 12-minute English/Spanish video labeled every word as English, even though large stretches were Spanish.

Steps

1. Batch transcription of a 12-minute English/Spanish video labeled every word as English, even though large stretches were Spanish. A maintainer clarified this is expected behavior: Speechmatics supports a single language per file, and the auto option just samples the audio to pick the predominant language for the whole file. The expected_languages option only helps determine which language model to use, not per-word language labels. A bilingual Spanish/English pack was planned for mixed audio.
2. The auto language option picks one predominant language for the whole file, so mixed English/Spanish audio will come back labeled as whichever dominated the samples. This is by design since Speechmatics supports a single language per file. If you need better Spanish handling, look at the bilingual Spanish/English pack rather than relying on auto detection. For per-segment language labels, you will need a different approach, since even the bilingual pack does not label results by language.

When to use

You are seeing this: Batch transcription of a 12-minute English/Spanish video labeled every word as English, even though large stretches were Spanish. Use this skill when you run into "Auto language detection returns English only for mixed English/Spanish audio".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Question asked in the thread: Batch transcription of a 12-minute English/Spanish video labeled every word as English, even though large stretches were Spanish. A maintainer clarified this is expected behavior: Speechmatics supports a single language per file, and the auto option just samples the audio to pick the predominant language for the whole file. The expected_languages option only helps determine which language model to use, not per-word language labels. A bilingual Spanish/English pack was planned for mixed audio.

Verified answer: The auto language option picks one predominant language for the whole file, so mixed English/Spanish audio will come back labeled as whichever dominated the samples. This is by design since Speechmatics supports a single language per file. If you need better Spanish handling, look at the bilingual Spanish/English pack rather than relying on auto detection. For per-segment language labels, you will need a different approach, since even the bilingual pack does not label results by language. Source: https://github.com/speechmatics/speechmatics-python/issues/80

Thread: https://vectle.com/threads/thr_CsHMpOKKKdZ7s021MtQohQ