...
If it’s an inline language change, such as just one sentence within a paragraph, you’ll need to wrap it in a <Span> tag first, and then apply the language change to the Span tag.
Open the Tags Pane.
Right click the tag that matches the content with the language change.
Select Properties.
In the Objects Properties box, select the Language from the drop down menu and click close.
Section 4.26: Changing the Language for Inline Text
Click the Selection Tool.
Then select the whole paragraph.
Open the Options Menu in the Tags Pane. Select Create Tag from Selection.
In the New Tag Box, Select Paragraph from the drop-down menu. Click OK.
Open the Options Menu and select New Tag.
Select Span from the drop-down menu and click OK.
Select the inline text with language change.
Right-click the Span Tag, click Create Tag from Selection.
Drag and drop the Span Tag to the correct spot in the tag tree.
Right-Click the Span Tag, click Properties.
In the Tag tab, select the language from the drop-down menu and click close.
...
Section 4.27: Changing Adobe Acrobat’s Language
Import your document, spreadsheet, or presentation into Adobe Acrobat.
Click the File tab and select Properties.
In the Properties Dialogue Box, go to the Advanced Tab.
Select Language in the Reading Options section and select the language of your choice.
...