Language export event

• Added weight from core.
• Added LANGUAGE_EXPORT event for being able to extend language data to be exported.

Example implementation:

public function handleHidden(StaticExportExporterLanguageMetadataEvent $event){
    $data = $event->getData();
    $language = $event->getLanguage();
    $configurableLanguage = $this->getConfigurableLanguageEntity($language);
    $data['hidden'] = $configurableLanguage->getThirdPartySetting('hidden_language', 'hidden', FALSE);

    $event->setData($data);
    return $event;
}