The default system language on HTC devices is based on the country you buy the phone. You can easily set new languages. To change system language on HTC One do as follows:
1. Open "Settings" app (Drag down the notification bar and tap the Gear icon)
2. Scroll down and tap "Language & Keyboard" (The icon is a Blue A)
3. tap the first option which is "System Language"
4. Select the new system language