Do you have the correct kde-i18n-* language package(s) installed?
There is a bug in Slackware-10.2 and it does not install the
KDEI (International language support for KDE) packages even if
you select it during installation. Therefore, you will need to
select your package(s) and install it from the CD or Slack website.
Afterwards you may also need to change fonts in KDE.