There are localized values for EN locale. I’d recommend that you set localized values Login / Password for those keys.
Also, please check that you do not copy EN messages.properties files from cuba to your project (com/haulmont/cuba package). That is why fallback to EN messages does not work.
In addition, I’m going to publish an “Italian translation” add-on very soon, to avoid the cumbersome process of adding the message files manually every time (and avoid issues like this one: #43 )