One video, every language

The original audio plus Spanish and German dubs, as three selectable tracks of one MP4. Each language uses a different translation engine and voice provider, to show they mix freely:
documentary.mp4 gives documentary.multi.mp4 with the original audio first, under the input’s own language tag, then tracks tagged spa and deu. A player opens it on the original audio.

One file per language

For platforms that take one audio language per file, write a video for each:
Each input then has two outputs, …es.mp4 and …uk.mp4, and both are listed under the input in GET /projects/{id}/files.

Dub some languages, subtitle others

A translation without ttsOptions is not dubbed; use it in subtitle outputs. Here Spanish is dubbed and French is only subtitled:
A dub track in fr would be refused: French has no dub.