Unfortunately what you see is what happens with ambiguous dates. The code assumes US format by default. There does not seem to be an easy answer (apart from e.g. using
http://www.gmayor.com/ManyToOne.htm to create the PDF and the e-mail).
As the date is correct in the merge document, presumably through the use of a formatting switch on the merge field, have you considered reading the value from the merge document, while it is available, and write it to a string. You can then write the string to your e-mail body (and that will retain the format as it is in the document).
Code:
Dim strDate As String
Dim oFld As Field
Set oDoc = ActiveDocument
With oDoc
.MailMerge.ViewMailMergeFieldCodes = False
For Each oFld In .Fields
If InStr(1, oFld.code, "MeldeschlussJVA") > 0 Then
strDate = oFld.Result
Exit For
End If
Next oFld
End With