AFAIK, you can use lookup dictionaries only with Latin languages. For example, Russian or Arabic dictionaries won't work.
A workaround is the use of transliterated headwords. That's what Russian Kindle users did, when they created their Russian-English dictionary.
As for the inflections, they need to be explicitly defined by the dictionary author, because they cannot be automatically generated by kindlegen.
I.e. you'd need to find a Greek inflection list and add the
entries to the source file.
Unlike Mobipocket Reader, the Kindle doesn't support fuzzy searches. I.e. don't expect it to suggest a similar dictionary entry if it doesn't find the exact word.
As for changing the keyboard layout, there's a
Kindle hack for it, but I haven't tested it myself.