but space is not changed
How do you know? You can't "see" a space character, so it has no color.

The code looks good. I rewrote the Sub Baset for speed. I hope I put all the lines in the right loops.


I added some code lines to place a double underline on all Spaces worked on. See the Comments to see how to remove them after testing. Run this sub and look for double Underlines where Spaces are. 
Sub Baset_PowerPoint_Language()

Dim sld As Slide
Dim shp As Shape
Dim L As Long

For Each sld In ActivePresentation.Slides
  For Each shp In sld.Shapes
  
    If shp.HasTextFrame Then
      'Using "Wtih" is faster
      With shp.TextFrame.TextRange
        .Font.Name = "Arial"
        .Font.NameComplexScript = "Arial"
        .ParagraphFormat.TextDirection = ppDirectionRightToLeft
        .RtlRun
          
        For L = 1 To .Characters.Count
          If CharType(.Characters(L)) = "English" Then
            With .Characters(L)
              .Font.Color = vbRed
              .LanguageID = msoLanguageIDEnglishUS
            End With
          
          ElseIf CharType(.Characters(L)) = "Space" Then
            If CharType(.Characters(L - 1)) = "English" And CharType(.Characters(L + 1)) = "English" Then
              'Do Space Stuff on Character L
              With .Characters(L)

                'X Line: Remove all X lines after testing
                .Font.Underline = -4119 'X line:  = Double under line
                'X line: Uncomment next line to remove underlines
                '.Font.Underline = -4142 'X line: = no underline

                .Font.Color = vbRed
                .LanguageID = msoLanguageIDEnglishUS
              End With
            End If
          End If
        Next L
     
      End With
    End If
  Next shp
  
  With sld.NotesPage.Shapes.Placeholders(2).TextFrame.TextRange
    .Font.Name = "Arial"
    .Font.NameComplexSc ript = "Arial"
    .ParagraphFormat.Te xtDirection = ppDirectionRightToLeft
    .RtlRun
  End With
Next sld

End Sub
Private Function CharType(Letter As String) As String
  Select Case AscW(Letter)
  Case Is < 48 'do nothing
  Case Is < 58 '0-9
    CharType = "English"
  Case Is = 32
    CharType = "Space"
  Case Is < 65 'do nothing
  Case Is < 91 '0-9
    CharType = "English"
  Case Is < 97 'do nothing
  Case Is < 123 '0-9
    CharType = "English"
  Case Is = 174
    CharType = "English"
  Case Is = 8482
    CharType = "English"
  End Select
End Function