It seems Valve and the many Steam gamers did not have a very merry Christmas. A glitch in the system caused many people to see other users' accounts instead of their own.
Multiple users reported seeing different accounts when they went into the Account Details page on their Steam app. Others were also reporting a language other than their default on the store's homepage.
For a while it wasn't clear whether this was a glitch at Valve's end or something more nefarious like a hack. Turns out though, it was the former. After fixing the glitch, Valve later informed Gamespot that it was a caching issue as a result of configuration change earlier in the day, which caused users to see other users' cached pages.
Valve assured that no unauthorized actions were taken and users could only see the page and no additional action is required on the user's end. But while that issue may have been solved, some users are still experiencing issues purchasing items. Your mileage may vary.