It could be because you have a Mac. I know that LanguageID wasn't supported in earlier Mac VBA but I thought it was fixed in newer versions. You may need to work with TextRange2 like this

Sub toSpanish()
Dim osld As Slide
Dim oshp As Shape
For Each osld In ActivePresentation.Slides
For Each oshp In osld.Shapes
If oshp.HasTextFrame Then
oshp.TextFrame2.TextRange.LanguageID = msoLanguageIDSpanish
End If
Next oshp
For Each oshp In osld.NotesPage.Shapes
If oshp.HasTextFrame Then
oshp.TextFrame2.TextRange.LanguageID = msoLanguageIDSpanish
End If
Next oshp
Next osld
 End Sub