When I have to set up new O365 tenants in order to do PoC or even to do large migrations, it might be convenient to change the default mailbox language settings for all the end users. By default each user needs to set the default language and time zone at first login to OWA in Office 365, if not you can find some problems due to this feature
With the following PowerShell Script you should be able to change it within a few seconds. Change it accordingly.
get-mailbox | Set-MailboxRegionalConfiguration -LocalizeDefaultFolderName: $true -DateFormat d/M/yyyy -Language 1027 -TimeZone "W. Europe Standard Time"
You can find the Local ID in the following web
That’s all folks!