Thanks for your fast reply sbillard.
I'm actually using the latest version of zenphoto and zenpage with a fresh install, my first one.
In the admin-optinos.php at line 226 I have this code:
$result = setlocale(LC_ALL, $locale.'.'.$encoding, $locale);
btw all the locales with te .po flies work flawless, I have only problems with en_us

Install the nightly build.

Great, latest nightly buld solved the problem in the admin-options!

On the other side now I can't set the front-end in english via the dynamic-locale plug-in. It works for fr, it and ge but not for english. After the usual warning it switches to the browser language, unless I set english instead of HTTP Accept Language in the back end.

I'll manage it somehow, anyway thanks for your great work on this project!

No guarentees if the locale functionality is not implemented on your server. You need to get that fixed.

I'll ask my provider. I'm quite new to this world so I don't understand if it's possible that locale is not enabled while I get warnings only with en_us and not with other languages.
Even during installation process, done in italian, I don't get any warning about this.
The only warning I get is:
Setting mbstring.internal_encoding to UTF-8 in your php.ini file is recommended to insure accented and multi-byte characters function properly.

When I tested zenphoto locally on my own PC I was unable to get locale working on windows, so I tryed with Ubuntu and everything went fine. In windows locale didn't work also with other languages, as one can expect..

A least some Windows OS support locale functionality: http://www.science.co.il/Language/Locale-Codes.asp

Hi again, I found a workaround, too simple and too dirty to trust it but i'll explain it here.
Before going on let me say that I asked my provider (shellrent.com, or their office in Italy) about the support of en_US but their answer was really evasive: "the locale is on, at least you can use it in Italian" :(
While I was waiting for a new answer to a more specific question about the same problem, I made this simple experiment:

1. I deleted the locale folder de_DE. I don't need it atm and German locale was working on my server.
2. I renamed the folder en_US to de_De
3. In functions-i18n.php I replaced the string:
   'de_DE' => gettext('German (Germany)'),
   With:
   'de_DE' => gettext('English (United States)'),

I know it's crazy but everyting works as needed now.
Feel free to remove this post if it can lead to improper use of your CMS by other people. This is non compatible with whatever upgrade of course.

ciao
bic

edit:
Of course the AcceptLanguage doesn't work in this way

After a few tests I discovered that also AcceptLanguage partially works. If you have de_DE as preferred language in your browser the gallery will be displayed in english. The second choiche in your browser will be ingored in this case.
But if you have en_US as first choiche your browser will display the second choiche, if any.

I suppose you could make an 'English US' translation....

LOL, after I finish the 'English - Spaghetti" one

Bingo! Finally my provider enabled en_US. Thanks again sbillard for you help, before talking with you I had no idea what arguments to use with them :)