This is a guide howto change the language of all power point slides to ... Found this in https://msdn.microsoft.com/en-us/library/aa432635.aspx

ChangePowerpoint language

ry the following steps:-

Step 1:- By using Macro we can change the language in powerpoint for all slides.

Create a new macro:
1. Go to Tools, Macro, and 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
Step 2:- This "msoLanguageIDEnglishUS" which is used in the above macro can be replaced by any desired language. The full list of languages can be found in this article http://msdn.microsoft.com/en-us/library/aa432635.aspx

Step 3:- 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.