<?php
// if we are in context www-en…
if ($modx->context->get('key') == "www-en") {
    $o = '';
    // …grab www-fr infos
    $modx->switchContext('www-fr');
    $ss = $modx->getOption('site_start');
    $ctx = $modx->context->get('key');
    // grab datas from TV named multi (containing ID of the translated resource)
    $tvv = $modx->resource->getTVValue('multi');
    // if there is no translation (empty TV), return the start_start ID
    if (empty($tvv)) {
        $o = $ss;
    } else {
        $o = $tvv;
    }

    $u = $modx->makeUrl($o,$ctx,'','full');
    // going back to original context
    $modx->switchContext('www-en');
    return $u;
}
// else if we are in context www-fr…
else if ($modx->context->get('key') == "www-fr") {
    // …grab www-en infos
    $modx->switchContext('www-en');
    $ss = $modx->getOption('site_start');
    $ctx = $modx->context->get('key');
    $tvv = $modx->resource->getTVValue('multi');

    if (empty($tvv)) {
        $o = $ss;
    } else {
        $o = $tvv;
    }

    $u = $modx->makeUrl($o,$ctx,'','full');
    // going back to original context
    $modx->switchContext('www-fr');
    return $u;
}
// we are somewhere else…
else {
    // …return an error (enhance later for > 2 lang)
    return '#error';
}​