Where it is: Settings → AI Behavior — reply in the visitor’s language, reply language, cross-language search, and the knowledge base language.
Multilingual replies are on by default. Someone writing in German gets German, without any configuration.
With WPML or Polylang
Content is tagged with its language as it is indexed, and a page in Dutch answers from Dutch content. Run one full sync after installing a translation plugin so existing entries get their language stamp.
Different alphabets
A question typed in Bengali or Arabic against an English knowledge base normally finds nothing, because it matches neither the vectors nor the keywords. The plugin translates such a question into your content’s language for searching only — the reply still comes back in the visitor’s language.
Forcing one language
Turn multilingual replies off to pin every answer to your site language.