Hi Efthimios,
This sounds to me like a number of smaller but related issues, rather than just one issue.
First: you are correct that currently, the English field labels are hardcoded into the finding aid templates. It is possible to translate them, however, in fact, our community has done so previously to create French versions of these. You can find the French versions, along with instructions on how to swap the English XSLT files for the community-created French ones here:
Hopefully this will give you a sense of how you could translate the XSLT files into Greek, and swap in the new files. If you do create a Greek version of the PDF and RTF XSLTs, please feel free to share them, and we will be happy to put a copy on the wiki for others to use!
Without having yet tested them myself, the encoding and the missing creator name both sound like bugs to me. The creator name is likely a culture fallback issue - you might try flipping the interface to English, adding an English translation of the name if possible, and seeing if that will at least allow it to display properly (or vice versa, depending on the current culture of the related authority record).
As for the encoding issue - I recall this being an issue with the default font used in Apache FOP. Unfortunately I no longer recall the reason, but our developers felt we could not include a different font by default in the AtoM release. However, we do have some instructions here on how you could use a different font with better support for special characters, glyphs, accents, etc. See:
Regards,