Redirect user to prefered language after login
You can use the hook_user_login which is called after a user successfully log in to the website. This hook passes the user object as an argument that you can use to retrieve the user language.

For the languages, you can make use of the language_list, which returns an array with the defined languages in your Drupal website. And for the redirect, drupal_goto does the job.

Here is some sample code that could help you further:

function YOURMODULE_user_login(&amp;$edit, $account)
{
    // retrieve user language
    $user_language = $account-&gt;language;

    // don't do anything if no language is defined
    if (!$user_language) {
        return;
    }

    // verify if language exists
    $languages = language_list();
    if (!isset($languages[$user_language])) {
        return;
    }

    // verify if language is enabled
    if (!$languages[$user_language]-&gt;enabled) {
        return;
    }

    // redirect to preferred language homepage
    drupal_goto('&lt;front&gt;', array(
        'language' =&gt; $languages[$user_language],
    ));
}