My website become something else for a few hours.

At 3pm (GMT +8), I accessed my website and everything looked okay.
At 5pm I opened again, it looked totally different. It showed Japanese words, with a lot of links that when I check, they were links around my website’s pages, but with a /zh/ extension (for Chinese translation)

Right now it’s 6pm, I cleared my cache and everything is back to normal again.

Between 3 and 5pm I did not change/update my website at all.

At the moment my website is back.
Could you please check if there’s anything wrong?
Can I know what is the cause of the problem as well?

The page I need help with: [log in to see the link]