Testé sur MS PowerPoint 2003 :

1) Va dans le menu "Outils > Macro > Macros "
2) dans le champ "nom" de la fenetre, tape "change_language_to_french"
3) clique sur le bouton créer
4) défini le code de la macro avec le code ci-dessous
5) Exécute le code avec F5

Sub change_language_to_french()
'
' Macro créée 07/04/2007 par Gilles Foucault
'

' Declare variables.
Dim sld As Slide
Dim shp As Shape

' Loop through all the slides in the presentation.
For Each sld In ActivePresentation.Slides

' Loop through each shape on each slide.
For Each shp In sld.Shapes

' If the Shape is a text box...
If shp.Type = msoTextBox Or msoPlaceholder Then
If shp.HasTextFrame Then

' ...then change the language to US English.
' NOTE: To change the language ID to another language,
' change the msoLanguageID value here to a
' different language.
shp.TextFrame.TextRange.LanguageID = msoLanguageIDFrench


End If
End If
Next
Next

End Sub