Range.LanguageID (Word)

Returns or sets a WdLanguageID constant that represents the language for the specified range. Here you can find possible values for WdLanguageID.

Some of the WdLanguageID constants may not be available to you, depending on the language support (U.S. English, for example) that you've selected or installed.

ActiveDocument.Paragraphs(2).Range.LanguageID = wdFrench 
Set myDictionary = CustomDictionaries.Add(Filename:="French.dic") 
With myDictionary 
 .LanguageSpecific = True 
 .LanguageID = wdFrench 
End With