Does this work?
Option Explicit
Public Sub ChangeSpellCheckingLanguage()
Dim j as Integer
Dim k as Integer
Dim scount as Integer
Dim fcount as Integer
scount = ActivePresentation.Slides.Count
For j = 1 to scount 
   fcount = ActivePresentation.Slides(j).Shapes.Count
      For k = 1 to fcount
         If ActivePresentation.Slides(j).Shapes(k).HasTextFrame Then
            ActicePresentation.Slides(j).Shapes(k).TextFrame2.TextRange.LanguageID = msoLanguageIdSpanish
         End If
     Next k
Next j
End Sub