Hello Panayiotis και καλησπέρα...

All those omissions exist because at this time the Greek language pack is not being actively maintained. The "obvious" candidate to do that would be, umm, me

, but unless the time machine goes into commercial production very soon there's no way I can work on code and language at the same time.
You can do it yourself though, and help all the Greek Moodle users at the same time! If you visit your admininstration menu, "Διαχείριση -> Διαμόρφωση -> Γλώσσα" you 'll find an "Edit strings" option that allows you to edit your language files from within Moodle.
After editing the language files, it would be very nice if you can send them back to us so we can include them in the greek language pack. In that case, you would probably want to contact the translation coordinator,
Koen Roggemans, at
translations@moodle.org for tips and instructions. He knows that stuff better than anyone else.

Also, you will find that it's better if you discuss issues like this one over at the Languages forum. That way, your posts will be read by people who are interested specifically in Moodle's language packs and you will get a better response rate.
If there is any matter that you need help on, please give me a private message on moodle.org or send me an email.
Γιάννης