On this search page, the inlanguage parameter is displayed to me as “⧼advancedsearch-field-inlanguage⧽” instead of “Pages in this language”.
The message MediaWiki:advancedsearch-field-inlanguage exists, so I’m not sure why this is happening. But if this form is assembled client-side, then perhaps there’s some ResourceLoader setup missing to make sure that message is also available in JavaScript?