Like a lot of open-source programs, R comes with the highly annoying “feature” of being extremely localized. Not only does it default to your system’s language, but also it makes it a pain in the neck to manage to change the language: using the “GUI preferences” menu (or whatever it is in your locale, haha ^^) to change the language doesn’t even seem work… (you need to restart R for a language change to apply, but when you restart your custom configuration doesn’t get loaded – awesome)
But, well, someone asked the question on an R mailing list hosted by the University of Newcastle, Australia, and the solution is to run R with the added command line parameter
LANGUAGE=en (edit the shortcut to R and add this stuff at the end).
On a side note, the solutioner points out a very common mistake made by localizers:
“Apparently some users want Windows in their native language, but not R.”
Dear localizers, you appear to forget or ignore the fact that some people run your software on their work computer, where they don’t have the choice of their OS nor of their language. You also seem to ignore that in some countries it’s pretty hard to grab a legal non-local copy of Windows. So please, don’t make it hard for users to change your program’s language back to normal (i.e. English). How would you like it if I forced you to read a Google-translated version of this notepad in your local language, hm?
Update (2012-01-16): as mentioned in the comments, an alternative, more radical solution is simply to delete the
share/locale folder. It contains translation files, and if R can’t find them it will fall back to English.