Murat Gunyar blogged about mailbox language settings in Exchange Server 2007. How it used to work and what changed with Exchange Server 2007.
Here is a summary on the mailbox language settings following the various tests that I could do and articles that I have read. I hope that this can help you.
How it worked before Exchange 2007
In previous versions of Exchange Server, the language for a user mailbox was determined by the language setting of the MAPI agent (such as the Outlook client) and not by the Exchange server:
- At the first connection of a MAPI agent (like Outlook client) to a user mailbox, the language of the default folders (like inbox) and system messages (like quota messages and non-delivery reports) was set on the information store (PR_LOCALE_ID MAPI property) to the language used by the MAPI agent.
- Each new connection of an MAPI agent changed the PR_LOCALE_ID of the mailbox to match the one used by the agent. Only system messages were modified, the default folders remained in the language of the first agent connection.
And now with Exchange 2007
In Microsoft Exchange Server 2007, you can specify the language preferences for a mailbox, in order of preference, with the Languages parameter by using the Exchange Management Shell and the Set-Mailbox cmdlet. Several Exchange components display information to the user using the preferred language, if that language is supported. These components include quota messages, non-delivery reports (NDRs), the Microsoft Outlook Web Access user interface, and Unified Messaging (UM) voice prompts.
Read more at source: http://msexchangeteam.com/archive/2009/01/28/450532.aspx