I needed a script to set the mailbox language and also change the DefaultFolderNames to that language. This can be done by a nice one-liner
Get-Mailbox "MailboxName" | Set-MailboxRegionalConfiguration -LocalizeDefaultFolderName:$true -Language 1031 -TimeZone "W. Europe Standard Time"
You have to change “MailboxName” to the name of the mailbox. In this case it will change the maibox default language to German (Language 1031 – see this page from Microsoft for all language codes), but if you change that to the language code of your needs, it’ll change the DefaultFolderNames (Inbox, Sent items, Outbox, Deleted items etc) to the selected language. If you want the time zone to be a specific one (in this case “W. Europe Standard Time”, you can check out this page from Microsoft with all names for the different time zones)