What Niggaphil said is correct. There are certain schools that teaches Mandarin to indonesian students, usually as 'additional' language that you've learned alongside Indonesians (primary, mostly for writings) and English (ESL in nutshell). Personal experience-wise, from my elementary school up till my college and subsequent drop-out from it, I have never ever encountered any Mandarin language teachings in any shape or form beyond extracurricular stuff.
That said, one of my family member who are currently in her college is also learning Mandarin through one of its course and I do remember other language being given throughout my years growing up for anyone to learn aside of ID and English. (I remember Japan being the popular one for weeb-ness and Arabian due to one of my school being a muslim-centric one, but also German and Russian... weirdly enough). And also, despite the enmity and racism toward the Chinese that happened during the 90s during economic crash and the end of Indonesian's "New Order" regime, there are still sizeable chinese folks that remained in Indonesia which at least enough to have people setting up chinese-focused learning centre and other things, like described above.
I can hazard a guess that Kobo ended up learning it at some point, probably during her younger years to point of being fluent to some degree. Maybe through those chinese-indonesian focused centre, or maybe her previous school forgoes English for Mandarin as first 'foreign' language to learn.