Gets existing translation language codes of the job item source.
Returns language codes that can be used as the source language for a translation job.
\Drupal\tmgmt\JobItemInterface $job_item: The job item.
array Array of language codes.
Overrides SourcePluginBase::getExistingLangCodes
public function getExistingLangCodes(JobItemInterface $job_item) {
// @todo Support additional source language codes.
return [
$this
->getSourceLangCode($job_item),
];
}