You would have to parse the Internet message headers to look for that.
Depending on the sending server you might see a header something like
"Content-Language". That can vary though.
Except for Outlook 2007 using the new PropertyAcessor object the object
model doesn't expose the headers. You have to use a lower level API such as
CDO 1.21 or Redemption (
www.dimastr.com/redemption) or Extended MAPI for
that. Since you don't say what version of Outlook or language or platform
you're using it's hard to say anything more.