Note that turning off "Detect language automatically" (and "Automatically switch keyboard to match language of the surrounding text") would not fix a document where there is already a problem with the language formatting. Try modifying the language of the Normal style in the affected document(s). If that fails, you will have to manually select all the text and apply the desired language via Review | Language | Set Proofing Language (exit the dialog box by clicking the OK button rather than the Set as Default one).

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional