I wonder, how do I force the Windows client to use English as the language and keep it between updates? Deleting the other language files works, but having to do so each update, and they come often, is not a great user experience.
You can set the language manually by editing the nextcloud.cfg file in your user profile folder. Look for the [General] section and add or update this line:
language=en
That should keep the client in English even after updates.