Maybe you can create a custom plugin that runs on the event “OnManagerLogin” and sets the correct language.
Something like this may work:
<?php
$eventName = $modx->event->name;
switch($eventName) {
case 'OnManagerLogin':
$id = $user->get('id');
if ($id == 1){ // user has ID = 1
$_SESSION['manager_language'] = 'en';
}
}