Sub ChangeProofingLanguageToEnglish()
    Dim j, k, m, scount, fcount, gcount 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
            If ActivePresentation.Slides(j).Shapes(k).Type = msoGroup Then
                gcount = ActivePresentation.Slides(j).Shapes(k).GroupItems.Count
                For m = 1 To gcount
                    If ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m).HasTextFrame Then
                    ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m) _
                    .TextFrame.TextRange.LanguageID = msoLanguageIDEnglishUS
            End If
                Next m
            End If
        Next k
    Next j
End Sub

 Sub ChangeProofingLanguageToEnglish()
    Dim j, k, m, scount, fcount, gcount 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
            If ActivePresentation.Slides(j).Shapes(k).Type = msoGroup Then
                gcount = ActivePresentation.Slides(j).Shapes(k).GroupItems.Count
                For m = 1 To gcount
                    If ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m).HasTextFrame Then
                    ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m) _
                    .TextFrame.TextRange.LanguageID = msoLanguageIDEnglishUS
            End If
                Next m
            End If
        Next k
    Next j
End Sub

The existing answers work for text that is present in the outline. Unfortunately in my case this didn't cover a significant part of the text, including figures, tables, etc.

This macro solved the problem for me :

Sub ChangeProofingLanguageToEnglish()
    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

The "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

(Credit goes to Ganesh Kumar who posted the original macro here)

The existing answers work for text that is present in the outline. Unfortunately in my case this didn't cover a significant part of the text, including figures, tables, etc.

This macro solved the problem for me :

 Sub ChangeProofingLanguageToEnglish()
    Dim j, k, m, scount, fcount, gcount 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
            If ActivePresentation.Slides(j).Shapes(k).Type = msoGroup Then
                gcount = ActivePresentation.Slides(j).Shapes(k).GroupItems.Count
                For m = 1 To gcount
                    If ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m).HasTextFrame Then
                    ActivePresentation.Slides(j).Shapes(k).GroupItems.Item(m) _
                    .TextFrame.TextRange.LanguageID = msoLanguageIDEnglishUS
            End If
                Next m
            End If
        Next k
    Next j
End Sub

The "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

(Credit goes to Ganesh Kumar who posted the original macro here. I added support for first level of shape grouping. To further improve it the macro can be made recursive to look for groups which contain other groups, etc.)