Hello there, 

thank you for your message. 

Unfortunately, Greek is not available for WebSite X5 15, I am very sorry about this. I kindly suggest you to create an Idea post to suggest the introduction of this language. 

Please also consider that you can add a new language if you need to use Greek as Content Language. To do this, you will first need to provide the translations of all the texts, so make sure you have these available when you want to add a new language. Once you have the necessary translations, adding a new language is easy:

• In the Language Content Management, click on the Add a new Language button and type "EL - Greek" in the dialog box that is shown. This new entry will appear at the bottom of the list of available languages.
• Click on "EL - Greek" to display the column for this language in the translation table.
• Type in the translations for each item and then close the window when you have finished: this automatically saves your work.
• Set "EL - Greek" as Content Language in the Website Settings window.

I hope this can be useful for you. Kind regards.