Turn multilingual captions on or off. Host only.

On 2026-10-12 a live meeting needed three refreshes to start transcribing its second language, because the stream decides its language when it OPENS and nothing could change a stream that was already running. This writes activity_config.multilingual (True, or False so it also beats a class that is multilingual by default), which every stream opened from now on reads, and tells everyone already in the call with a server-sent data packet, {"type": "captions_mode", "multilingual"}. Each browser then switches its own caption socket in place (change_language), so nobody refreshes and no audio is dropped.

Open breakouts get the same flag and the same packet, as same-room mode does. A room whose languages include one outside nova-3's multi set (Chinese) is refused with 409 rather than silently left monolingual.