For mac users, I found this on the R for Mac FAQ
If you use a non-standard setup (e.g. different language than formats), you can override the auto-detection performed by setting `force.LANG' defaults setting, such as for example
defaults write org.R-project.R force.LANG en_US.UTF-8
when run in Terminal it will enforce US-english setting regardless of the system setting. If you don't know what Terminal is you can use this R command instead:
system("defaults write org.R-project.R force.LANG en_US.UTF-8")
but do not forget to quit R and start R.app again afterwards. Please note that you must always use `.UTF-8' version of the locale, otherwise R.app will not work properly.
This helped me to change my console language from Chinese to English.