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Using Powerpoint 2010 I opened the Outline menu -

Selected all text (Ctrl+A), opened the language menu and set my proofing language

And it worked!

The language menu is located on the Review ribbon tab (after the Slide Show tab and not visible on the screenshot).

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.)

I improved upon Inigo's answer to provide a recursive version that changes all items to the desired language.

This version will recursively investigate each shape that is a group type. Some experimentation suggests that msoGroup and msoSmartArt are the group types - feel free to add to that list if you find other types of shapes that can hold text objects.

Sub ChangeProofingLanguageToEnglish()

    Dim j As Long, k As Long

    Dim languageID As MsoLanguageID



    'Set this to your preferred language

    languageID = msoLanguageIDEnglishUK



    For j = 1 To ActivePresentation.Slides.Count

        For k = 1 To ActivePresentation.Slides(j).Shapes.Count

            ChangeAllSubShapes ActivePresentation.Slides(j).Shapes(k), _

              languageID

        Next k

    Next j

End Sub





Sub ChangeAllSubShapes(targetShape As shape, languageID As MsoLanguageID)

    Dim i As Long



    If targetShape.HasTextFrame Then

        targetShape.TextFrame.TextRange.languageID = languageID

    End If



    Select Case targetShape.Type

        Case msoGroup, msoSmartArt

            For i = 1 To targetShape.GroupItems.Count

                ChangeAllSubShapes targetShape.GroupItems.Item(i), languageID

            Next i

    End Select

End Sub

Based on Inigo, Duncan, Maria and DomDev's answers, this works for shapes, tables, groups, SmartArt, now and in the future:

Sub ChangeProofingLanguageToFrench()

    Dim j, k As Integer

    Dim languageID As MsoLanguageID



    'Set this to your preferred language

    languageID = msoLanguageIDFrench



    'Loop all the slides in the document, and change the language

    For j = 1 To ActivePresentation.Slides.Count

        For k = 1 To ActivePresentation.Slides(j).Shapes.Count

            ChangeAllSubShapes ActivePresentation.Slides(j).Shapes(k), languageID

        Next k

    Next j



    'Loop all the master slides, and change the language

    For j = 1 To ActivePresentation.SlideMaster.CustomLayouts.Count

        For k = 1 To ActivePresentation.SlideMaster.CustomLayouts(j).Shapes.Count

            ChangeAllSubShapes ActivePresentation.SlideMaster.CustomLayouts(j).Shapes(k), languageID

        Next k

    Next j



    'Change the default presentation language, so that all new slides respect the new language

    ActivePresentation.DefaultLanguageID = languageID

End Sub



Sub ChangeAllSubShapes(targetShape As Shape, languageID As MsoLanguageID)

    Dim i As Integer, r As Integer, c As Integer



    If targetShape.HasTextFrame Then

        targetShape.TextFrame.TextRange.languageID = languageID

    End If



    If targetShape.HasTable Then

        For r = 1 To targetShape.Table.Rows.Count

            For c = 1 To targetShape.Table.Columns.Count

                targetShape.Table.Cell(r, c).Shape.TextFrame.TextRange.languageID = languageID

            Next

        Next

    End If



    Select Case targetShape.Type

        Case msoGroup, msoSmartArt

            For i = 1 To targetShape.GroupItems.Count

                ChangeAllSubShapes targetShape.GroupItems.Item(i), languageID

            Next i

    End Select

End Sub

The version of Duncan works well for everything but tables.
I found another code which seems to also work with tables: https://stackoverflow.com/questions/4735765/powerpoint-2007-set-language-on-tables-charts-etc-that-contains-text

Public Sub changeLanguage()

    On Error Resume Next

    Dim gi As GroupShapes '<-this was added. used below

    'lang = "English"

    lang = "Norwegian"

    'Determine language selected

    If lang = "English" Then

        lang = msoLanguageIDEnglishUK

    ElseIf lang = "Norwegian" Then

        lang = msoLanguageIDNorwegianBokmol

    End If

    'Set default language in application

    ActivePresentation.DefaultLanguageID = lang



    'Set language in each textbox in each slide

    For Each oSlide In ActivePresentation.Slides

        Dim oShape As Shape

        For Each oShape In oSlide.Shapes

            'Check first if it is a table

            If oShape.HasTable Then

                For r = 1 To oShape.Table.Rows.Count

                    For c = 1 To oShape.Table.Columns.Count

                    oShape.Table.Cell(r, c).Shape.TextFrame.TextRange.LanguageID = lang

                    Next

                Next

            Else

                Set gi = oShape.GroupItems

                'Check if it is a group of shapes

                If Not gi Is Nothing Then

                    If oShape.GroupItems.Count > 0 Then

                        For i = 0 To oShape.GroupItems.Count - 1

                            oShape.GroupItems(i).TextFrame.TextRange.LanguageID = lang

                        Next

                    End If

                'it's none of the above, it's just a simple shape, change the language ID

                Else

                    oShape.TextFrame.TextRange.LanguageID = lang

                End If

            End If

        Next

    Next End Sub

In addition to answer provided by Mastergalen and to address comments regarding newly type text:

If you will notice, that language will automatically change back whenever you start to type new text (which is very annoying), you have to change current default language for PowerPoint:

• make sure PowerPoint window is an active window


• in the Windows Taskbar (yes, actually not in PowerPoint), check if Language bar is visible,
  
  
  
  
  • if not go to Control Panel > Region and Language > Keyboards and Languages. Click Change keybords..., switch to Language bar tab and check Docked in the taskbar option. (this is from Win7, so might be a bit different in other versions).
  
  
  
  


• now key action - in the Language bar in the taskbar, click language code and switch to EN (if you want currently to use English in PowerPoint). From now on, all new text in PowerPoint will be in the selected language :-)


• if you want write in your original language, just change it back.

I made an add-in back in 2014 for myself which still works fine in PowerPoint 2016. https://github.com/wobba/officeaddin/releases/tag/v1.0.1

It scans for used languages, and allows you to change all at once, looping over.