Update Zend_Validate.php resource

The current status of `Zend_Validate.php` resource is plain broken in pretty much every language.
This is an attempt to quickly fix the current state (#2536), ignoring the proposal of adding full stops a the end of each message which, as discussed in #2984, has raised some BC concerns.

At the moment this PR completely fixes EN and IT languages only, but I'll try to fix the other languages too, at least the message keys.