If you rely on "Detect language automatically," note that it takes a while to kick in. In particular, as you have seen, if you add text in different languages in one and the same paragraph, Word usually doesn't recognize the language correctly.

What you can do is create a character style with the language that you want and apply it to the relevant text ranges.

__________________
Stefan Blom
Microsoft Word MVP

Microsoft 365 apps for business
Windows 11 Professional