I was plain wrong.
Of course there is no po file for k3b, neither in kde-l10n-fr-4.10.2 nor in /k3b-2.0.2_20120226.git shipped in Slackware-current.
In fact my memory didn't serve me well that time. Your question reminded me of a similar one I answered on LQ but that I couldn't find at first. It's this one
But Amarok is not k3b!
I investigated a bit more and with the help of http://packages.slackware.com/
saw that there *was* a k3b-i18n package in slackware, but that was not shipped after Slackware-12.2
So yes I think you're right, this looks like a packaging problem on Slackware's side, as upstream source do include a po/ directory
Maybe there is a reason for omitting it, but in any case I'll drop a line about that to Pat, just in case he doesn't read this thread.
Oh and you didn't need to pull the sources from git, there is a tarball
(but may be you did that because the source tarball for k3b in Slackware is pulled from git as well.)
BTW, I wrote that Slackware usually package released versions and actually that's not the case for k3b. Well, as the saying goes: exceptio probat regulam