You're free to try and translate it again. We have another user that's having issues translating a file, similar to yours. They sent us their INI files and we noticed the encoding was different than we normally encode our language files. We re-encoded them and sent them back to the user and are awaiting a response.
If that works, we'll help re-encode your files into the proper UTF-8 format, but not sure if that's the cause. For now, if you want to test, try:
1) Copying the en-GB file to your file. Do not edit immediately.
2) See if the translation works properly (really, you shouldn't see a difference)
3) Rename the original en-GB file to something else. What this is for is to ensure that the gl-GR file is actually being used (and not the fall back to en-GB)
4) If it appears the el-GR file is being used, try to edit it and make some changes. See if the strings stop working.
If they stop, that's a sign that the encoding isn't correct.
Either way, let us know how it goes, and we'll keep you posted with what we find from the other user.
Thanks,
Alex