All you need is to find somebody with same watch as yours and ask him to sent you Greek language pack.
He need to connect his watch to computer, open as USB drive and in Primary/ Garmin / Text he will find that language.
If you do the same with your watch, you can past this new language. Just one more step - you need to delete one of your language pack and rename greek.ln4 to that supporter language. Now, if you want greek, just change to that renamed language.
It works!