When you open up R do you see errors like the following appear in red:
1: Setting LC_CTYPE failed, using "C"
2: Setting LC_COLLATE failed, using "C"
3: Setting LC_TIME failed, using "C"
4: Setting LC_MESSAGES failed, using "C"
5: Setting LC_PAPER failed, using "C"

To fix this on Mac OSX do the following:

  1. Close R
  2. Open Terminal
  3. Write or paste in: defaults write org.R-project.R force.LANG en_US.UTF-8
  4. Close Terminal
  5. Start R