When the first time you turn on your Windows smartphone, there should be a language selection page at the start-up settings just like the screenshots below. If you unfortunately entered to a non-English system by any chance, please follow the steps below to change the system language to English.
First of all on the Start screen (main page), tap on the "All apps" arrow which is located at the bottom of the page, or simply just swipe over from the Start screen to get there.
Then, in the "All apps" list, tap Settings which is usually a gear wheel icon.
On the Settings page, scroll all the way down till "language" is found under the time+language section. If you can't really read the language, try to look for something like (UTC+08:00). "language" should be the one below that.
On the language page, tap on the plus sign (+) to "add languages", check one of the English options to add English and tap the check button at the bottom of the screen.
After English is added, tap the restart button to restart the phone. Microsoft requires a restart every time a language is added.