Set the document language in a PDF to allow screen readers to switch to the appropriate language. According to Adobe, this check determines whether the primary text language for the PDF is specified. If the check fails, set the language.
To set the language automatically, right-click "Primary Language" on the Accessibility Checker panel and then choose "Fix" from the Options menu. In the "Set Reading Language" dialog box, select the appropriate language and click the "OK" button.
To set the primary language manually, click "File" in the main menu and select "Properties." Click the "Advanced" tab and under "Reading Options" and set the document’s language in the Language drop-down box. Click the "OK" button to save your changes.
WCAG requirement: 3.1.1 Language of Page