Same problem here.

Build 9357 on Windows Server 2022

That error means that SmarterMail detected a possible folder name conflict between one of the default folders and a custom folder in the new language. Since the error message specifically indicates an email folder, that means there is another email folder with a name that would conflict with Inbox, Deleted Items, Drafts, Junk Email, or Sent Items. For example, if I created a folder called "Posteingang" and then tried to switch my language to German, SmarterMail would detect that as a conflict with the Inbox. To correct the issue, you'll need to identify the conflicting folder and rename or delete it.