OnePlus
Change System Language auf OnePlus
On OnePlus phones running OxygenOS, the system language gets locked in during the initial setup and never changes on its own afterward. This is controlled by the Languages & input function inside the system settings, which manages both the display language and the keyboard input options separately.
If your phone shows the wrong language, or you simply prefer a different one, you do not need to reset the device or go through setup again. Just reorder the language list so your preferred language sits at the top, and the system applies it instantly across menus, apps, and notifications.
Menu path to change the system language
Open the Settings ➔ Tap on System ➔ Tap on Languages & input ➔ Tap on Languages (country/region) ➔ Tap on Add a language or hold a language ➔ Drag the language to the first position
If your desired language is not listed yet, use Add a language first, then drag it to the top of the list. Apps that only support a limited set of languages may still fall back to English or another available option, which is normal and not a bug.
A factory reset is only necessary if the language setting is fully unreadable due to a system error, which is rare and unrelated to a simple language switch.