A bug in the Mac GUI and a request

Sys.getenv("LANG") in the GUI, you should get something like:

Sys.getenv("LANG")

         LANG
"en_US.UTF-8"

Sys.getenv("LANG")

                      LANG
"en_GB at currency=EUR.UTF-8"

Wow, that's weird one. Can you, please, try to run

defaults write org.R-project.R force.LANG en_GB.UTF-8

in the Terminal and run the GUI again? Consequently UTF-8 should be  
working again. Please let me know whether it does and I'll fix the  
GUI to do that automatically. You can revoke the above by running

defaults delete org.R-project.R force.LANG

Thanks,
Simon