https://www.reddit.com/r/Steam/comme...team_websites/

If you know you're already logged in, do NOT visit any Steam Community or Steam Store URL.
This includes any internet browsers and the Steam Desktop/Mobile Client!
Playing games online should be fine. It'll also prevent anybody else from logging into your account via desktop client.

Do NOT unlink PayPal, do NOT remove credit card info from Steam's websites. You may choose to do that on external websites instead.

Explanation according to Steam DB:

Valve is having caching issues, allowing users to view things such as account information of other users.

This is also why the Steam website has been displaying in different languages.

Keep an eye on Twitter @steam_games or facebook.com/Steam for any official messages.