How to change Gmail language
If you are using Gmail interface in the non-English language then see the simple way to change to English here.
Details of the way to change the Gmail mailbox language
Gmail language change
Step 1: You access the following link: https://mail.google.com/mail/u/0/#settings/general
Or you press the gear on the top right corner of the Gmail window, select Settings. Settings screen appears, select the General tab.
Step 2: In the Language click to change your language, select English (US) as shown above.
If you want to switch to other languages then do the same.
Step 3: Scroll down to the bottom and click Save change button.