well, unfortunately I found that although the latest version of Polylang plugin was successfully installed after I reset the data in the database, post translations data were lost though. I tried to manually update the plugin version by version, but the result was still the same (the problem occured after upgrade from version 1.1.6 to version 1.2).
well, I had lost days messing around trying to find the problem :) so finally I gave up and assigned every post translation manually via WordPress GUI :)