Debian 7 - wheezy
To check which locale is currently set as default just run:
$ locale
or type to see a list of installed locales
$ locale -a
then run
$ dpkg-reconfigure locales
and choose your language as default
then type
$ export LANG=nl_NL.UTF-8
export LANG=(two letter code for your language)_(two letter code for your country).UTF-8
reboot and check again your new locale
$ locale
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