I also have a HTC that does NOT have Chinese as a system language option when I wanted to change the system language from English to Chinese. Following HTC's guide was not usefully as Chinese language is not even an option: https://www.htc.com/cn/support/h ... splay-language.html
I previously changed my system language to simplified Chinese language, but I don't exactly remember how I changed it initially. Any how, I now want to change simplified Chinese system language to traditional Chinese system language. I found the following worked for me to make this change.
I am not sure if this works using other browswers not by Google LLC, but it works for me from my Google Chrome browser download onto my HTC One M9: