It makes absolutely no sense for advertising to switch all advertising to Spanish from a single day of recording. This would mean they disregarded ALL of the meta data they had on them. Location, things they visited, pages they visited etc. I've been on vacation and spoken a different language for two weeks and it didn't change the language of my ads. It just makes no sense to do that from a single data point, when all else contradicts them being/speaking Spanish.

There is a lot of misinformation on what Facebook is and isn't doing. And a lot of it is pushing 10 years old.

Facebook has long had features that detect exactly what you're describing. They aren't recording it, they are fingerprinting it. The target is any ads and music that is played but it could go beyond that.

This is fundamentally no different than the way a device is passively listening for the "hey, assistant" phrase which just matches a fingerprint.

Anyone who is simply looking for immediate data transfer when this occurs is a fool. There is absolutely no reason it cannot hold the list of known finger prints and add them to otherwise normal requests. The same for anyone looking for cpu spikes; these fingerprints are highly performant and it's not recording, it's matching so Facebook can deny all day that they don't record your conversation and it isn't a lie because it's the wrong accusation.

It's much easier for apple to have shared the data that your friend watched Telemundo for 24 hours and thus either has a friend with them that speaks Spanish or is learning Spanish

Or for the Facebook app on their phone to have noticed another app get installed with those details

Its not the microphone