Hello Michael_RH,
Good morning!
Thank you for publishing in Microsoft Community. We are happy to help you.
Thank you for sharing the steps you have taken to resolve the issue with the language detection feature in Outlook. Since the online repair tool did not work, If your Outlook isn’t spell checking, grammar and spelling mistakes won’t be flagged, or correctly spelled words may come up as incorrect. We need to check some information.It's important to know the type of account you're using, because each account has a different process and a different support category.What type of account is affected? is outlook.com account, Hotmail account, or professional/educational account.However, to better understand the problem and provide a solution, we need more details from you, please provide us with the following information. Thank you.1-Please can you share with us the affected outlook version screenshot as below picture:

2-Please Check if the spell checker is correctly activated
Once you’ve restarted Outlook, it’s a good idea to check the settings for outlookautocorrect. You can open the menu as follows:
- Go to the “File” tab.
- Click on “Options” in the left-hand menu.
- Switch to “Mail” in the pop-up window.
- Select “Spelling and AutoCorrect”.
From the options that are listed below “When correcting spelling in Outlook” check the box next to “Check spelling as you type” and “Mark grammar errors as you type”.
From the options that are listed below “When correcting spelling in Outlook” check the box next to “Check spelling as you type” and “Mark grammar errors as you type”.

When the options to “Check spelling as you type” and “Mark grammar errors as you type” aren’t activated, spell check will not be working in Outlook. Close and Restart Outlook
3-Please, Start composing a new email in the usual way. Open the “Review” tab and click the “Language” button. Select “Set proofing language” to view current language settings for spell and grammar checks.

Once you’ve set the desired language, test the spell check manually by pressing the “Spelling & Grammar” button in the “Review” tab. The spell checker launches a small dialog window that should display the correct language. Remove the checkmark next to “Detect language automatically” (if there is one), as this can lead to issues, for example, if you tend to use foreign words in your emails.

4-Please can you install the office app in the affected Pc and reinstall. Then check the issue
We look forward to your reply. This requested information will help us narrow down the scenario and find the right troubleshooting. We will continue to assist you based on the information you provide. I sincerely appreciate your patience and cooperation.SincerelyEben Ezer Tres | Microsoft Community Moderator