Have posted about this before, but doing a cleaner post on it.
SInce 4.2 the privileges for setting locale changed and you either need a rooted a phone or grant the privileges with tools from a computer.
This is done on a regular HTC One, it’s not rooted or changed in any way and the process shown here will not alter anything on the magnitude like root.
First you need to hook up phone to computer with USB debugging enabled (or it will not work very well). This you do in Developer options if it’s not on already, if you can’t find developer options: read this