https://www.reddit.com/r/Games/comments/3y7maa/something_is_really_wrong_with_steam_be_careful/

There are reports of people seeing other user's personal and account information, being logged in as another user, etc. Best thing to do is to NOT TOUCH steam whatsoever if this is a cache overflow issue like reports suggest, and unlink your paypal account from steam if you have one. Watch your bank account transactions during the coming weeks and if there are any odd transactions, report it to your bank These are just precautions because Valve hasn't disclosed the full extent of the issue yet.

I was wondering why my steam store page was in Russian....

Merry fucking Christmas - Valve