Thanks for the access to the site. I'm not sure what was wrong with your el-GR.com_jfbconnect.ini file, but I renamed it to el-GR.com_jfbconnect_orig.ini, and then the en-GB file started working again.
The en-GB fallback only works if the current language file can't be found. Since it could, that file was being used and Joomla doesn't 'fall back' if it finds the right file. However, it seems like Joomla was having an issue parsing that file, causing the issue. Not sure what was wrong with it, I did a quick look and things seemed correct. Possible issues are invalid characters in the strings, incorrect comments characters (should be # or ; if I recall correctly) or improper encoding.
Either way, it looks to be fixed now. Please let us know if you're still seeing issues though.
Thanks,
Alex