Bilingual Site Language Problem

My site is in English and Greek. When I set up a new product listing and do the title and description in both English and then in Greek, sometimes I find that the one of the languages has suddenly lost its own title and description and adopted the title and description of the other language.

This has also happened on older listings, suddenly I find they are no longer in both languages but the same in both.

Can anyone advise me as to why this happens and how to stop it?