hello

i have 3 languages site EN,PL and DE
and in CMS 404 error page for each language

here are the names of this pages:

EN - page-not-found
PL - nie-znaleziono-strony
DE - seite-nicht-gefunden

but i always get EN 404 error page, no matter if i am on english,polish or german version

also in source of error_page i have

            <% if Locale == "en_US" %>
               <% include ikony_en %>
            <% end_if %>

            <% if Locale == "de_DE" %>
               <% include ikony_de %>
            <% end_if %>

            <% if Locale == "pl_PL" %>
               <% include ikony_pl %>
            <% end_if %>

but it doesnt work too

what is the reason ?

The problem is that we don't have enough information in the URL to determine which language to show the 404 page in.
As of 2.4 alpha1, this works:

http://mysite/unknown-page // shows in english (assuming its the default locale)
http://mysite/unknown-page?locale=de_DE // shows in german

Of course, you can't rely on visitors setting the right locale when accessing pages.
There's no built-in way support for getting a locale from different TLDs (mysite.de, mysite.com, etc),
or from pseudo sub-urls (mysite.com/de/unknown-page), which would provide this information.
This is a known limitation, see http://open.silverstripe.org/ticket/3877

Another approach would be to inspect the "Accept" language in HTTP headers in Page_Controller->init()
and redirect appropriately.

If anybody wants to attempt a core patch for this, submissions are welcome
Pointers: ModelAsController->getNestedController(), Translatable->contentcontrollerInit(), ErrorPage::response_for()