any i386, i486, or i586 distro will work on your pentium...
kde 3.0 will work, but that's a very old version, and it might suck...
also, kde is very resource-hungry... it would probably be very slow with those specs...
you might wanna use someting lighter, such as xfce, icewm, fluxbox, etc...
http://www.xfce.org
http://www.icewm.org
http://www.fluxbox.org
yes, c and c++ are completely supported... there's PLENTY other supported languages...