Everybody has seen it one or twice. When importing exchange data using PST files that a user has both an postvak in as an inbox folder.
I found a nifty tool that changes the language of the exchange mailbox to a specified language.
For example :
As you can see the above mailbox language is in English.
Now you start the tool (ignore the mapi error) and select the user(s) you wish to change :
You hit set and you will notice the mailbox(es) are changed into the preferred language :