conditional (theme) language selection at ‘runtime’

I found a solution, realy dirty, but works.

In the core.php file of the polylang plugin I found this stuff:

// NOTE: I believe there are two ways for a plugin to force the WP language
// as done by xili_language and here: load text domains and reinitialize wp_locale with the action 'wp'
// as done by qtranslate: define the locale with the action 'plugins_loaded', but in this case, the language must be specified in the url.
function load_textdomains() {
    // sets the current language

    if (!($this->curlang = $this->get_current_language()))
    return; // something went wrong

    // --- modified --> check for guestbook, which should be display in different languages
    $this->check_joined_content();
    ...
}

define("GUESTBOOK_PAGE_ID", "12");
define("GUESTBOOK_HTTP_PARAM", "guestbook_lang");

function check_joined_content() {
    $gl = $_GET[GUESTBOOK_HTTP_PARAM];
    if(isset($gl)) {
        if ($this->curlang->object_id != GUESTBOOK_PAGE_ID) {
            return;
        }
        if ($gl == "de") {
            $this->curlang = $this->get_language("de");
        } else {
            $this->curlang = $this->get_language("en");
        }
    }
}

access to the pages via http get params:
?guestbook_lang=en and ?guestbook_lang=de