How do I download more languages ​​in SIRI;

Hi. The languages displayed in Siri Settings are the only languages supported yet.

No, Greek is not one of the languages Siri currently supports:

About Siri - Apple Support

Sorry it wasn't the answer you wanted.

Submit your feedback directly to Apple requesting they add Greek using the appropriate link on the Feedback page:

http://www.apple.com/feedback

Use the link I provided.