That shouldn't be the problem then, since both are UTF-8.

Continuing to look  at this Alec.

Paul

A.L.E.C wrote: 
Paul Horton wrote:

  
It's also possible to call setlocale() at run time to alter PHP's 
interpretation of data.
    

So, roundcube uses such code:

     // set localization
     setlocale(LC_ALL, $_SESSION['language'] . '.utf8', 'en_US.utf8');

     // workaround for http://bugs.php.net/bug.php?id=18556
     if (in_array($_SESSION['language'], array('tr_TR', 'ku', 'az_AZ')))
       setlocale(LC_CTYPE, 'en_US' . '.utf8');

I'm using pl_PL.utf8, and en_US.utf8.