The first report came yesterday and it read:
Yesterday everything was fine, but today when I go to www.google.com/webmasters everything is in Spanish, since I do not speak or read Spanish this is a problem. I've searched but can't seem to find any answers on why this is happenning or what to do to change it back to English.
A few other webmaster tools users said they experienced the same thing. It worked in English the day before and then when they tried it yesterday, it took them to a Spanish version.
A fix, which Cristina posted in the forums was to append hl=en to the end of the URL string, after you are logged in. That fixed it for everyone, but why did it switch over in the first place?
Google has not yet chimed in on the thread, so I am not sure how big or small of an issue this is.
Forum discussion at Google Webmaster Help.