I have installed dutch and the standard english and I have forum in dutch and in English but isn't it possible to select a forum based on your language automatically because now I have 2 usergroups and if they don't select a usergroup they see a forum mixed english with dutch.
Are there any solutions for this problem?
Not without code modification. You can mod.
config.php to select different databases (mybben, mybbde) depending on user's browser preffered language thus having two separate forums, e.g.:
$lang=strtolower(substr($_SERVER['HTTP_ACCEPT_LANGUAGE'],0,2));
if ($lang!='de') $lang='en'; //Force English if browser not configured for Deutsch.
$config['database'] = 'mybb'.$lang;
Ofcourse, $lang could be set with cookies, refferers or variables (from your main site) and so on - it's just an example. Make your check depending on desired languages.