add a file called e.g greek.ufo to base/ufos (if the ufos folder doens't exist, create it)
fill with content like this:
language gr {
"_Greek"
{
gr_GR.UTF-8
gr_GR.utf8
}
}
i'm not sure about the gr_GR stuff - add all possible locale strings to this list. then compile your po file into a mo file (poedit will do this for you)
after you have that mo file compiled, go into base/i18n/ and create gr/LC_MESSAGES copy your greek ufoai.mo file (maybe you have to rename it - it must have the name ufoai.mo) into base/i18n/gr/LC_MESSAGES/ufoai.mo
next time you start the game it should be listed in the options menu.
please also submit the gr.po file to our patch tracker - as i've already written in your other post. then we can add it to the svn and release it officially with the next patch
(edit: gr.po, of course, not fr.po)