What language does Feitan speak when activating Pain Packer? In Hunter x Hunter, episode 97, Feitan activates his nen ability.

What language is the vertical text on the right?
According to the Hunter x Hunter Wiki:


  Feitan's first language is Chinese; he speaks in sentence fragments in Japanese while switching to his native tongue when angered.


Given this, and the text is written in either Japanese kanji or Chinese (as @kuwaly beat me to pointing out), and the fact that there is a Japanese translation provided, I'm going to conclude that the text is written in Chinese. (There may be some fragments of Japanese, but I somewhat doubt that.)