You could do both in any of these ways:
1. Do not translate the title and put a footnote/endnote with the translation (mentioning the fact that it has not been translated into Greek).
2. Translate the title and put a footnote/endnote with the original translation (mentioning the fact that it has not been translated into Greek).
3. Do not translate the title and add a translation in parenthesis within the text.
Usually, the publishing house has a policy regarding footnotes/endnotes (which may differ depending on whether the book is fiction or non-fiction) and they should be consulted.