Selection.LanguageDetected (Word)

Returns or sets a Boolean that specifies whether Microsoft Word has detected the language of the selected text.

Check the LanguageID property for the results of any previous language detection. The LanguageDetected property is set to True when the DetectLanguage method is called. To reevaluate the language of the specified text, you must first set the LanguageDetected property to False.

With Selection 
 If .LanguageDetected = True Then 
 x = MsgBox("This document has already " _ 
 & "been checked. Do you want to check " _ 
 & "it again?", vbYesNo) 
 If x = vbYes Then 
 .LanguageDetected = False 
 .DetectLanguage 
 End If 
 Else 
 .DetectLanguage 
 End If 
 If .Range.LanguageID = wdEnglishUS Then 
 MsgBox "This is a U.S. English document." 
 Else 
 MsgBox "This is not a U.S. English document." 
 End If 
End With