I have knowledge articles created in
en_US in my org and want to query in what other languages the article has been translated
A way to fetch what other languages it has been translated to is to filter on the knowledgearticleid or Id in your soql.
As per the SOAP API Developer Guide documentation on KnowledgeArticleVersion:
Always filter on a single value of Language. However, in SOQL, you can filter on more than one Language if there is a filter on Id or KnowledgeArticleId.
SELECT Title, language FROM KnowledgeArticleVersion WHERE PublishStatus='Online' AND knowledgearticleid='kA0f40000004Wq7CAE'