Doesn't recognize locale properly

[Uqbar]

My current locale settings are:
LANG="en_US.UTF-8"
LC_NUMERIC="it_IT.UTF-8"
LC_TIME="it_IT.UTF-8"
LC_MONETARY="it_IT.UTF-8"
LC_PAPER="it_IT.UTF-8"
LC_NAME="it_IT.UTF-8"
LC_ADDRESS="it_IT.UTF-8"
LC_TELEPHONE="it_IT.UTF-8"
LC_MEASUREMENT="it_IT.UTF-8"
LC_IDENTIFICATION="it_IT.UTF-8"

as defined in /etc/default/locale and

LC_COLLATE="C"

in my ~/.bashrc

That is: I need all messages in Englishwhile using local (Italian) cenventions for numbers, time etc and use "C" collation.
Everytime I run apt-get I get this error message:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
        LANGUAGE = "en",
        LC_ALL = (unset),
        LC_TIME = "en_IT.UTF-8",
        LC_MONETARY = "en_IT.UTF-8",
        LC_COLLATE = "C",
        LC_ADDRESS = "en_IT.UTF-8",
        LC_TELEPHONE = "en_IT.UTF-8",
        LC_NAME = "en_IT.UTF-8",
        LC_MEASUREMENT = "en_IT.UTF-8",
        LC_IDENTIFICATION = "en_IT.UTF-8",
        LC_NUMERIC = "en_IT.UTF-8",
        LC_PAPER = "en_IT.UTF-8",
        LANG = "en_IT.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_ALL to default locale: No such file or directory

As you can see, the locale is recognized improperly as "en_IT" instead of "it_IT".
I would expect no such mangling.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: apt 1.0.1ubuntu2.6
ProcVersionSignature: Ubuntu 3.13.0-44.73-lowlatency 3.13.11-ckt12
Uname: Linux 3.13.0-44-lowlatency x86_64
ApportVersion: 2.14.1-0ubuntu3.6
Architecture: amd64
Date: Wed Jan 14 08:46:47 2015
InstallationDate: Installed on 2015-01-12 (1 days ago)
InstallationMedia: Kubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: apt
UpgradeStatus: No upgrade log present (probably fresh install)

[Uqbar (uqbar)]

The problem is getting worse now.
My /home filesystems has historically been formatted with XFS and I've been using UTF8 encoding since long now.
A few files there sport non-ASCII characters in the name, mainly a few accented vowels from my home language (Italian).
I have recently reinstalled the whole system (XFS+UTF8) and have restored the /home contents from a straight backup (another XFS+UTF8 file system) with "cp -a" command.
Everything seemed to work fine until I run a simple "ls -l" in a directory containing one of those files.
That name gets displayed with "??" instead of the accented vowels.
A software referring to another of those files says it cannot access it any more.
The bash doesn't allow me to enter those accented vowels any more.
I think the "locales" systems is behaving badly.

[Uqbar (uqbar)]

The actual values for those environment variables are the same as reported by apt:

$LC_ADDRESS = en_IT.UTF-8
$LC_COLLATE = C
$LC_IDENTIFICATION = en_IT.UTF-8
$LC_MEASUREMENT = en_IT.UTF-8
$LC_MONETARY = en_IT.UTF-8
$LC_NAME = en_IT.UTF-8
$LC_NUMERIC = en_IT.UTF-8
$LC_PAPER = en_IT.UTF-8
$LC_TELEPHONE = en_IT.UTF-8
$LC_TIME = en_IT.UTF-8

[Uqbar (uqbar)]

man -k locales
man: can't set the locale; make sure $LC_* and $LANG are correct