I use the English version of LireOffice, but I would also like to download the Greek version. Can I download 2 seperate versions or will the Greek version replace the English version?
You can install a Greek language pack (pick the right platform and then click on Translated User Interface: ελληνικά), and then switch from one UI language to the other on Tools → Options → Language Settings → Languages → User interface (the program needs to be restarted).
Note that you can also create a dedicated user profile for another UI language, create a shortcut for using that profile, and then use the two profiles simultaneously (their settings will be independent).
Also note that installer for Windows already includes all UI languages - you only need to run the installer again, and make sure that all UI languages you need are checked.