It is very easy to change in the sources, but I am not sure it is a good thing. Because there are many users with /usr/share/fet/translations now. FET should be installed under GNU/Linux with "sudo make install", so with root access. I could modify so that FET searches in share, then in local, but also I am not sure it is a good thing, because it mixes too many things.
But why can't you put the translations in the fet.app?
The responsible code is in src/interface/fet.cpp, search for "share" (without quotes).