Does anybody know an English speaking University (in any country) which would accept a PhD thesis written in student's mother language? (for example , is it possible to study for a PhD in a US university (or any other country in the word) and to write my dissertation in Greek language)?

No - the only exceptions I could think of might be possibly if the thesis dealt with modern Greek literature and was defended in a Modren Greek Dept or if on ancient Greece and written in ancient Greek at a very traditional university and even then I imagine you'd have to have a very good explanation. Why not do a PhD at a Greek or Cypriot university?