Setting the document language in a PDF enables screen readers to switch to the appropriate language. 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, select Primary Language in the Accessibility Checker tab, and then choose Fix from the Options menu. Choose a language in the Set Reading Language dialog box, and then click OK.

To set the language manually, do one of the following:

Choose File > Properties > Advanced and then select English from the drop-down list in the Reading Options section. This setting applies the primary language for the entire PDF.
Set the language for all text in a subtree of the tags tree. Open the Tags panel. Expand the Tags root and select an element. Then choose Properties from the Options menu. Choose a language from the Language drop-down list. (To display the Tags panel, choose View > Show/Hide > Navigation Panes > Tags.)
Set the language for a block of text by selecting the text element or container element in the Content panel. Then, right-click (Windows) or Ctrl-click (Mac OS) the text and choose Properties from the context menu and choose a language from the Language drop-down list. (To display the Content panel, choose View > Show/Hide > Navigation Panes > Content.)

Run the Make Accessible Action Wizard to help guide you through this step much faster.