Check the output of
locale -a
If the locale that SVN is complaining about isn’t installed, then you can install it.
You might need to do for Debian or similar systems:
sudo dpkg-reconfigure locales
If you want to configure locales manually:
sudo vim /etc/locale.gen # and add "en_US.UTF-8 UTF-8"
sudo locale-gen
Or if your locale-gen supports an argument (NOT for Debian):
sudo locale-gen en_GB.UTF-8
sudo locale-gen en_US.UTF-8
Alternatively as Ankit writes in his answer:
export LC_ALL=C
may work (in your current session, or in your .profile).