The issue was i've installed WAS 8.5.5 only with EN language on a IT server and IBM installation Manager was set in Italian.
To fix it , we could modify the followin file
<Installation Manager folder>\eclipse\configuration\config.ini
and add osgi.nl= with new language code
In my case i've insert osgi.nl=en and after restart with IBM IM has started in English and I was been able to complete the setup.
Here the IBM technote about IM language change.