Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
I suspect that the problem has no good solution: In my experience (and I have done extensive research on several cases of this kind) a stereo or radio intended for the Japanese market, does not have capability to change languages from Japanese.
(There is also a related issue, in these case, that makes me ask simply: Can you read Japanese? if not, then even if I do discover a possible way to change language in your model the Pioneer C9P6, it may have to be done by blind navigation techniques, through the menus on your stereo, if you cannot read Japanese kanji.)
This might help:
See whether the stereo has a MENU メニュー option (Menu) has a 設定 (Settings) option and within it, a 言語 (Language) option. If it does, then you can try to switch between the default 日本語 (Japanese) and 英語 (English).
For 'C9P6' I find nothing but a model of Chromebook. The model # for the Pioneer stereo, from the manufacturer's label, may be needed, for me to be sure of the details on this specific stereo. If you find out that information, please let me know, and I will continue to advise you. Thanks.