if you are about to install qtranslate for the first time you should consider using qtranslate-x instead of qtranslate which seems to be discontinued.
As for the installation of the greek language follow these steps (they are pretty much the same steps in both plugins qtranslate and qtranslate-x ):
1. go to Settings > Languages and at the bottom of the page find the List of Configured Languages with the little flags.
2. If you see there the Greek Language just press ‘Enable’. If you don’t see the Greek Language then go to ‘Add Language’ on the left and choose:
Language Code el
Flag gr.png
Name Ελληνικ?
Locale el_GR
Date Format %d/%m/%y
Time format %H:%M
Not Available Message “Συγγν?μη,αυτ? η εγγραφ? ε?ναι διαθ?σιμη μ?νο στα %LANG:, : και %.”
hope it helps.