[Felipe]

Its very easy, you just need edit the config.languages.php file and change to your prefered default language and theme, you need to change the values of

$default_theme = 0; //index of theme, starting with zero
$default_language = 5; //index of language, starting with zero

Generaly this file stay at:

/home/admispconfig/ispconfig/web/webmail/inc/config.languages.php

Good luck.

Felipe Rocha