Exchange 2007 mailbox users with incorrect language settings

Users may report that their Exchange mailbox displays in an incorrect foreign language in Outlook and Outlook Web Access.  This issue may occur when the Languages attribute of the mailbox is incorrectly set.

You can view the language of a mailbox using the Get-Mailbox cmdlet.

[PS] C:\>Get-Mailbox -Identity "John Smith" | fl name, languages

Name      : John Smith
Languages : {en-AU}

To see all mailboxes that do not match the language you are expecting you can use this PowerShell command, where “en-AU” is the expected language in this example:

[PS] C:\>Get-Mailbox | where {$_.languages -ne "en-AU"} | fl name, languages

Name      : Peter James
Languages : {en-US}

Name      : Frank Wu
Languages : {zh-CN}

To set a user mailbox to the language you desire use the Set-Mailbox cmdlet.

[PS] C:\>Set-Mailbox -Identity "Frank Wu" -Languages "en-AU"

Link: How to change the languages for a user mailbox