In which I walk you through changing the language settings on all the things!
In an attempt to improve my German I wanted to switch the default language of websites and software I use. Also a friend had just switrched their Facebook over for the same reason and I wanted to give that a whirl. Here's how to do that for some common websites/programs.
There is more info on the effectiveness of language immersion in this reference.
If you want a small amount of immersion Facebook is a good place to start. Your friends' postrs etc. will all still be in your native language so it is mostly the settings and menus that will teach you things. Probably most helpful for dates, times and variations on the words 'comment' and 'like'.
Changing Chrome is probably less useful than changing Facebook but at least you learn the words for 'saving' and all the variations on how to open a tab. Also worth noting many extensions change language based on the Chrome setting so YMMV.
Actually a lot more useful than the previous two. Especially useful if you are using the suite of Google apps on a regular basis.
Changing your OS display language is probably going to give you the most mileage. I have only covered the two versions of Windows I had to hand.
In some versions of Windows 7 you can use Windows update for language packs. In others you will need to download them from Microsoft. Instructions for both methods are in the Windows Help.
Language packs are now downloaded directly through Windows.
See above. You probably use your phone OS an awful lot and so maximum immersion.
Android 7 Coming Soon!
 Cummins, J. (1998). Immersion Education for the Millenium: What We Have Learned From 30 Years of Research on Second Language Immersion? In M. R. Childs & R. M. Bostwick (Eds.) Learning through two languages: Research and practice. Second Katoh Gakuen International Symposium on Immersion and Bilingual Education. (pp. 34-47).