http://www.linuxquestions.org/questions/debian-26/perl-warning-falling-back-to-the-standard-locale-c-208709/

mritch is correct.

you can also do it by:

#dpkg-reconfigure locales

choose your locale(s)

for example i choose all the en_US and the es_ES utf-8

in the next screen choose the one you want for your 'environment'
wait for to generate the locales and voilá