How to change the language of your Debian system
* First, you have to set environment variables such as LANG, LANGUAGE, LC_CTYPE, LC_MESSAGES to your local language. Usually LANG (or LC_ALL) is sufficient. (shamedlessly stolen from Claws Mail doc 
Check what you have at presente with: #env | grep LANG * Second, you may also need to reconfigure your locales.
To do this, as root:
First: issue on a root terminal #export LANG=(the two letter code for your country).UTF-8 That is, for the Spanish language it would be:
# export LANG=es_ES.UTF-8
Second: reconfigure locales issuing the following command:
# dpkg-reconfigure locales
A window will ask you to select the languages you want to have available. Choose your own.
Changes may not show inmediately, a reboot will be needed.
(Please feel free to modify these instructions if you know better. Thanks.)
How to change the language of KDE applications
Show available languages:( DebSearch:kde-i18n )
apt-cache search kde-i18n
Change the language:
apt-get install kde-i18n-thetwoletter (of your country)
as an example (for german 'de'utschland) :
apt-get install kde-i18n-de
Now you have your language in all KDE applications!