In Exchange 2010, there's a configuration parameter called "ExternalDsnLanguageDetectionEnabled"
In my situation, it meant that most of the error messages, the so called NDR's (Non Delivery Reports) or DSN's (Delivery Status Notification) was sent in Danish.
In an international Company, this is not what we want.
So I had to change the default language of the NDR's to be English instead of Danish.
This can be done by using the set-transportconfig command.
I did this:
get-transportconfig | set-transportconfig -ExternalDsnLanguageDetectionEnabled $false -ExternalDsnDefaultLanguage en-us -InternalDsnLanguageDetectionEnabled $false -InternalDsnDefaultLanguage en-us
So the autmatic language detection is turned of, and the default language is set to English
Please notice, in Exchange 2007, you can use set-transportserver instead.