When Exchange have to send a NDR message it will by default try to detect the language of the received message and send the NDR back in the same language.

Sometimes it will not be able to detect the language of the received message and therefore sends the NDR in the language that is default for the configuration.

To set the default NDR language you have to run a PowerShell command.

Use the following command to set the default language to English for NDRs

Exchange 2007

NDRs for mails received from external domains:

Set-TransportServer <hubservername> -ExternalDsnDefaultLanguage en-us

And for NDRs for mails received from internal domains

Set-TransportServer <hubservername> -InternalDsnDefaultLanguage en-us

The setting has to be set for every transport server in your exchange organization.

Exchange 2010

In exchange 2010 it only has to be set once for the whole Exchange 2010 orginization.

NDRs for mails received from external domains:

Set-TransportConfig <OrganizationIdParameter> -ExternalDsnDefaultLanguage en-us

And for NDRs for mails received from internal domains

Set-TransportConfig <OrganizationIdParameter> -InternalDsnDefaultLanguage en-us

The automatic detection of language for NDR can be disabled by running the following commands:

Exchange 2007

Set-TransportServer <hubservername> -ExternalDsnLanguageDetectionEnabled $false

and

Set-TransportServer <hubservername> -InternalDsnLanguageDetectionEnabled $false

Exchange 2010

Set-TransportConfig <OrganizationIdParameter> -ExternalDsnLanguageDetectionEnabled $false

and

Set-TransportConfig <OrganizationIdParameter> -InternalDsnLanguageDetectionEnabled $false

I hope this is useful.