"She eats a lot so not to become the thinnest."
Translation:Dia makan yang banyak agar tidak menjadi yang terkurus.
"biar" is the colloquial synonym of "agar", "supaya".
In the KBBi definition, you see "cak".
"cak" = ragam cakapan, ragam takbaku ==> only used in informal/colloquial style, not the standard.
That's the reason it's not used in this course, but it's perfectly fine to use it in everyday speech.