I was having this same issue in KDE on Ubuntu 14.04 and fixed it by editing ~/.kde/env/setlocale.sh and setting these values:
export LANG=en_US.UTF-8
export LANGUAGE=en
export LC_NUMERIC=en_US.UTF-8
export LC_TIME=en_US.UTF-8
export LC_MONETARY=en_US.UTF-8
export LC_PAPER=en_US.UTF-8
export LC_IDENTIFICATION=en_US.UTF-8
export LC_NAME=en_US.UTF-8
export LC_ADDRESS=en_US.UTF-8
export LC_TELEPHONE=en_US.UTF-8
export LC_MEASUREMENT=en_US.UTF-8