Private Function is_english_word(s As String) As Boolean
Dim i&
    
    is_english_word = True
    For i =  1  To Len(s)
      Select Case Mid(s, i,  1 )
        Case "a" To "z", "A" To "Z"
        Case Else
            is_english_word = False
            Exit Function
      End Select
    Next i
    
End Function


Private Sub Document_Open()
Dim Words_Count As Variant
    
    Words_Count = ActiveDocument.Words.Count -  1 
    For i =  0  To Words_Count
       With ActiveDocument.Words(i)
       
       If is_english_word(.Text) = True Then
          .LanguageID = wdEnglishUK
       End If
          
       End With
    Next i

End Sub