A family of Microsoft word processing software products for creating web, email, and print documents.
I have discovered a workaround for this bug.
In the Web version of Word and OneNote, When you select Immersive Reader, you can click on the Reading Preferences icon at the top right (the book icon), and set Translate to the language that the document is already in. By default it will turn on the "By Word" option - turn on the Document option as well. Now the Immersive Reader will read in the selected Language.
Note that this is a only workaround for a bug - the Immersive Reader in the web client is still failing to read text in the correct language as specified by the Proofing Language for that text block. And it doesn't work with documents that have multiple languages, because the whole document is translated to your "target" language. The Immersive Reader in the Web Clients is still broken.
The suggestion for the workaround came from this post: