just do a reinstall from thunderbird.net

I have no idea why this occasionally happens. But generally it is linked to your geo-location pointing to one locale and your installation being something else. A reinstall generally fixes things with no loss of data unless you actually delete the profile folder. Personally I do not recommend uninstalling, just reinstalling.

I updated with both the .msi and the exe. I am still seeing the mixed language.

This is urgent because I need to update my account password and I do not understand Dutch well enough to determine where that is done.

Do you have a Dutch language pack installed (under Languages in Add-ons Manager)? They are known to cause issues in 91, and are independent of the installed language version of TB, i.e. they are part of the profile folder.

No, only English (US). Thank you for the thought. I would appreciate it if you or anyone else who asks me a question, please tell me where to find it on the menu. While I can understand a little Dutch, most of the menu is beyond me. Addons is obvious but others may not be

Open Preferences (Voorkeuren), then Privacy & Security in the left pane (Privacy & Veiligheid), then in the Passwords (Wachtwoorden) section click the button Saved Passwords (Opgeslagen wachtwoorden), then Remove (Verwijderen) the entries with the old password. Restart TB and enter the new password when prompted.

Thank you so much - that worked. The menu remains mixed Dutch & English but at least it's not urgent now.

If the reinstall of the English version doesn't remove the Dutch, and there is no Dutch language pack under Tools/Add-ons & Themes, Languages (Talen), try creating a new profile by launching Profile Manager:

close TB, open a Run window (Winkey+R), type thunderbird.exe -p

I take it that I will still be able to access all my past emails. What will I be losing as a result of creating a new profile?

The current profile will be unaffected, and you can transfer mail and contacts from the old to the new. Either profile can be launched from Help/More Troubleshooting, about:profiles.

https://support.mozilla.org/en-US/questions/1354892#answer-1452751

Okay, I will do that this weekend. Thanks.

I rebooted my PC and Thunderbird came up in all English. Thanks for all your assistance and patience.