My Lumia 350 keyboard just change automatically to Arabic. How can I change it back to English keyboard. The phone uses windows 8 operating system.
On Win10 Mobile (I haven't used actual WP8 in some time), you can swipe the space bar left or right two switch between installed keyboards. This may work on WP8 as well.
You can (and probably want to) also remove the Arabic keyboard. Go to Settings -> Keyboard, and there should be a list with both English and Arabic. If you press-and-hold on Arabic, you should be able to uninstall it (you may need to have it not be the currently active keyboard, first). This will both prevent accidentally switching in the future, and free up some space in the phone's storage.