As mentioned in the documentation, Exchange Online does not care about the value of PreferredLanguage
: https://learn.microsoft.com/en-us/microsoft-365/troubleshoot/access-management/set-language-and-region
If you want to programmatically set the display language and folder localization for Exchange Online mailboxes, you have to use the Set-MailboxRegionalConfiguration cmdlet. Use the example in the article above, and add the -LocalizeDefaultFolderName
parameter to cater to folder names.