Yes, that is the intended behaviour. What is the issue?

The question is... why? If I start with a survey in language it I don't expect it to change after an upgrade of the application. The data in the database should remain as they are... or not? Is it written somewhere that upgrading form 1.92+ to 2.00+ the language it changes to it-informal?

Previously there was 'it' and 'it-formal'. We renamed it to 'it-informal' and 'it' to be consistent with other languages. If you have running invitation-only Italian surveys then rather wait until they run out before upgrading to 2.0

The problem is that nobody knows about this.
I figured out that something changed in languages because I couldn't use labelsets and lables created in 1.92+ after upgrading to 2.0+
I think that everything could be solved by changing labelsets and lables languages as well in the upgrade process.
I changed labelsets and labels language to it-informal in the MySQL database and everything seems to be fine.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=10276

Fix committed to 2.1 branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=10304

New version 2.00+ Build 121116 released