System Wide Configuration

Slackware has a profile script which allows you to set the system’s locale for all users at once (system-wide). You can simply edit this file with an ASCII editor such as nano or vi. Check out
/etc/profile.d/lang.sh

(users of C-shell and derivatives must edit /etc/profile.d/lang.csh instead).

If you want to be done with this quickly, just look for the line in that file which says:
# en_US is the Slackware default locale:
export LANG=en_US

and change the value to your own language. A finnish family would change this to
# fi_FI.utf8 is the Slackware default locale:
export LANG=fi_FI.utf8

That is not enough for desktop user, so you must make two files:
~/.bash_profile which determines .bashrc
~/.bashrc

nano ~/.bash_profile

The contents:
if [ -f ~/.bashrc ]; then
. ~/.bashrc # –> Read /etc/bashrc, if present.
fi

nano ~/.bashrc

The contents:
setxkbmap fi

Logout and login and finnish locales are working .. :)

If using KDE desktop, add autostart program via Settings -> System Settings -> Boot and Shutdown -> Add program -> setxbkmap fi to the box