@jeanhenry,
I've done that with Finnish and had a hard time reverting back.
You know on the right column where it says "trending" and underneath are some news headlines. Right underneath it says "English (US), Privacy, Terms, Cookies etc. In your case it might read "Chinese" or "Japanese" - regardless, click on the language and it will bring you to all others to choose from and from there you can get back to English.