R 2.1.0 GUI language

[...]

LC_ALL doesn't seem to change the message language. LC_MESSAGES,
which is mentioned  in the installation and administration guide,
doesn't seem to exist in R 2.1.0 and isn't listed in the locales help.

Right: it cannot be set inside R.  For as the manual says:

   Note that you should not expect to be able to change the language
   once R is running.

But it can be set as an environment variable outside R, as described in 
the rw-FAQ.  Setting either LC_MESSAGES or LC_ALL=de on the command line 
works for me.

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595