Brazilian Portuguese.

Funnily enough, it took me a while to determine it was definitely Brazilian Portuguese, given I'm a native Portuguese speaker, because the whole thing is written in a stiff academic-ish style that hides some the differences between European and Brazilian Portuguese.

> My Firefox detected it as English

Try selecting some text in the README, and right-clicking then "Translate to ..." and the autodetect might do a bit better to identify the language.

Looks like Portuguese to me (I don't speak it)