Did you also set i18n? Don't know if this may have any effect, but I always used them together and SiteTree also (somehow) uses i18n.
// Set the site locale and allowed languages
i18n::set_locale('en_US');
$allowed_locales = array(
'de_DE' => array('German', 'Deutsch'),
'en_US' => array('English', 'English'),
'ar_EG' => array('Arabic', 'العربية')
);
i18n::$common_locales = $allowed_locales;
// Make Pages translateable into other languages
Translatable::set_default_locale('en_US');
Translatable::set_allowed_locales(array('en_US', 'de_DE', 'ar_EG'));