Great conversation here!
I found out the reason –
There is a little quirk with detection for post language, a problem which solution has been eluding me. In the text you wrote in English but it has mentions of French. Depending on which LLM sometimes it would be detected as French or English. (In case you are curious, the prompt looks like this).
When a post is detected to be French, we skip translating to French – the first step of this feature (which is language detection) is very important and can cause confusion downstream if it’s wrong. 
As it was designed, if I write in “Indonesian” now (a language not supported on meta), it should still translate to English and all the other languages we support.
The site setting content_localization_supported_locales define the languages that any content can be translated to.
This is a great question and I’ve incorporated it into our FAQ:
My personal opinion is to write in what you’re comfortable with 
Turning on this feature on meta allows us to test and improve the feature and prompts.