Here is the case - you receive a presentation from someone else and find out that the default language for all text box is a foreign language while the context is English. When this happens, ydo not even try to reset the language (a default would only apply to new slides). Two possibilities offers itself:

Language setting option and outline view

Other possibility includes going to the outline view (second icon from the bottom left corner of a PowerPoint presentation in the XP version).  Then Ctrl-A to select all and go to "Tools" -> "Language"and choose your language to set. In some cases, slide titles may not be transformed for some reason.

The macro trick below works like a charm in all tested cases ... you may want to save such macro for future.

VBS solution

Here is a recipe however I found on the internet (in this thread ). The steps are pretty straight forward.

1. Go to Tools, Macro, Visual Basic Editor.
    
2. Insert a new empty module by selecting Insert, Module.
    
3. Paste this code on the right panel and save the macro:
   

   Option Explicit  
   Public Sub ChangeSpellCheckingLanguage()  
       Dim j As Integer, k As Integer, scount As Integer, 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
                   ActivePresentation.Slides(j).Shapes(k) _
                   .TextFrame.TextRange.LanguageID = msoLanguageIDEnglishUS
               End If
           Next k
       Next j
   End Sub
   
       

   msoLanguageIDEnglishUS can be replaced by any desired language. The full list of languages can be found here.

4. Execute the macro (by pressing F5 within the editor, or by selecting Tools, Macro, Macros, ChangeSpellCheckingLanguage, and clicking Run).

After that all text elements within the presentation will have the new spelling language. Neat.