You could use this translation:

$s_new_account_greeting_admincreated = 'L\'utente %1$s ha creato un\'utenza per te con nome "%2$s". Al fine di completare il processo di registrazione, visita il seguente URL (assicurati di digitarlo su una sola riga) e imposta la tua password personale d\'accesso:';

Seems that the italian translation is not up to date, there are more than 100 messages missing
Have a look at
http://translatewiki.net/w/i.php?title=Special%3ATranslate&task=untranslated&group=out-mantis&language=it&limit=150
If you want to contribute to MantisBT you can help by following these instructions
http://www.mantisbt.org/wiki/doku.php/mantisbt:translationshowto?s=translatewiki

ok, I'll do it!

Great to hear that you will do the job.

Be aware that there are also three standard plugins delivered with ManntisBT with own translations
http://translatewiki.net/w/i.php?title=Special%3ATranslate&task=untranslated&group=out-mantis-mantiscoreformatting&language=it&limit=100
http://translatewiki.net/w/i.php?title=Special%3ATranslate&task=untranslated&group=out-mantis-mantisgraph&language=it&limit=100
http://translatewiki.net/w/i.php?title=Special%3ATranslate&task=untranslated&group=out-mantis-xmlimportexport&language=it&limit=100

The translations in translatewiki are merged to MantisBT every few weeks, so your changes will become part of next versions of MantisBT, maybe 1.2.2

massimo_itaca, there was an update from translatewiki today
Your translations will be included in version 1.2.2
Thank you for contributing to MantisBT