I am a native English speaker and I have switched the Oppo X9000 to English under the Language settings; but whenever I...

I disassembled and reassembled the phone following the iFixit instructions, but now the power button is stuck and I cannot...

Most likely, you accidentally installed the button upside-down. If you install the button upside-down, it cannot make contact with the switch on the motherboard, and will not return to it's original unpressed length. To fix this, see the power button replacement guide.