After creating a mailbox it will be initialized when you access the mailbox for the first time with Outlook/OWA and default mailbox folders will be created in the language of the client.
The following options are available to change the language of the default mailbox folders after mailbox initialization:
1. User runs Outlook with parameter: outlook.exe /resetfoldernames
2. User changes default folder language from within OWA: Options, Regional Settings, Rename default folders so their names match the specified language
3. Admin (Exchange 2010 only): Set-MailboxRegionalConfiguration mbx -Language xx-XX -DateFormat "dd-MM-yyyy"-LocalizeDefaultFolderName:$trueLanguage examples: nl-NL, en-US