I saw a isDocumentTranslatable method which is not based on localeMapping but 
return !empty($metadata->translator)
            && is_string($metadata->translator)
            && count($metadata->translatableFields) !== 0;

Shouldn't be cleaner to do this : 
$metadata = $this->dm->getClassMetadata(get_class($document));
if ($this->isDocumentTranslatable($metadata)) {
}
?