hey dimitris,
installing these 2 files with the extension manager won't work. instead here's what you can do:
1) after you downloaded the 2 files arlex mentioned you should have now these 2 files on your computer:
for_use_komento_frontend_el_GR.ini and
for_use_komento_backend_el_GR.ini
2) the first file with the "frontend" in the name must be copied via ftp to your folder /language/el-GR (this will be the language file for the frontend later)
3) the file with the "backend" in the name must be copied to /administrator/language/el-GR (this will be the language translation for the admin area)
4) now in order for both language files to work you have to rename them (both) to the correct name, this would be: el-GR.com_komento.ini
5) that's it