For console fonts, I guess you need to load the correct module for locale support.
But for mozilla and other progs in xorg, you need to install truetype fonts (ttf) or other supported fonts.
What language are you looking for?
One of the ways to get the locale fonts is to get it from a Windoze machine. If the machine have been localise, it would contain the fonts in c:\windows\fonts. I think.
Just copy the fonts folder (zip it) and put it into Slackware, somewhere like ~/.fonts or other directory you wish.
If you put it at other path, make sure you include that path into /etc/fonts/fonts.conf.
You need to run fc-cache and restart xwindows.
Please correct me if I am wrong!