Everything is explained in the subject. There is no french localization for amarok-2.6.0. Can sombedy explains why it's missing ?
According to the upstream bug I found, it is because it was not ready. The translation appears to be present in 2.6.90 (2.7 beta).
According to the last comment on https://bugs.kde.org/show_bug.cgi?id=311328, the translation is almost done. I have tested and it's indeed the case. Can it be integrated in gentoo ? It's better than nothing.
I don't know if the maintainer will want to specially package it, given that 2.7 is supposed to be released soon. The 2.7 beta (which should include the translation) is in the KDE overlay if you wish to use the translation urgently.
2.7.0 is now in the tree, which again includes the French translation.
Unfortunately, this version can't be compiled because of a sandbox violation (see bug #453088)