Hello,
Since I've upgraded with Mac OS X El Capitan, I've only the english langage.
El Capitan locks the system.
Unfortunately, I cannot help you with advice.
It must work if you copy the fet_fr.qm file to the directory /usr/share/fet/translations, but this is not a good idea if you always want to use/edit the latest translation file, from the FET archive.
I tried to do it manually but El Capitan blocks the folder creating even with root's rights in /usr/share/ :-\
Try installing FET: "sudo make install", after "make" in the FET directory. This will install the translations in /usr/share/fet/translations.
You don't need to begin again with "make" if FET is already compiled - just go there and make install.