@faramirtr if you cannot find it in the menu, contact Oppo customer support
Hotline: 1800-767-6776
Monday to Sunday: 9:00 - 21:00
Email: service@oppo.com.sg
Seems to be a bit of a pain to change the language on this phone. Check [https://www.youtube.com/watch?v=upkmPBSqsTM|this video] as well as [http://www.hardreset.info/devices/oppo/oppo-r9/faq/language/|these directions]