Webmail in other language as in English: open

/var/www/ispcp/gui/tools/webmail/config/config.php 

and search

$squirrelmail_default_language = 'en_US'; 

and replace in your lang abbreviations are in the file locale example for german :

$squirrelmail_default_language = 'de_DE';

Filemaneger in other language as in English: open

nano /var/www/ispcp/gui/tools/filemanager/settings.inc.php 

and search

$net2ftp_settings["default_language"] 

and replace en in your lang. You find your language

/var/www/ispcp/gui/tools/filemanager/languages

Example for german: replace in

nano /var/www/ispcp/gui/tools/filemanager/settings.inc.php

and change

$net2ftp_settings["default_language"] = "en";

in

$net2ftp_settings["default_language"] = "de";

for German language