Option Explicit

'Déclaration des fonctions API
Private Declare Function GetLocaleInfo Lib "kernel32" Alias "GetLocaleInfoA" (ByVal Locale As Long, ByVal LCTYPE As Long, ByVal lpLCData As String, ByVal cchData As Long) As Long
Private Declare Function SetLocaleInfo Lib "kernel32" Alias "SetLocaleInfoA" (ByVal Locale As Long, ByVal LCTYPE As Long, ByVal lpLCData As String) As Long
Private Declare Function GetUserDefaultLCID Lib "kernel32" () As Long

'Déclaration de la constante séparateur décimal
Private Const LOCALE_SDECIMAL = &HE

Public Property Get DecimalSeparator() As String
Dim nLength As Long
Dim nLocale As Long
    nLocale = GetUserDefaultLCID()
    nLength = GetLocaleInfo(nLocale, LOCALE_SDECIMAL, vbNullString, 0) - 1
    DecimalSeparator = Space$(nLength)
    GetLocaleInfo nLocale, LOCALE_SDECIMAL, DecimalSeparator, nLength
End Property

Public Property Let DecimalSeparator(ByRef Value As String)
Dim nLocale As Long
    If Value <> DecimalSeparator Then
        If Value = "." Or Value = "," Then
            nLocale = GetUserDefaultLCID()
            SetLocaleInfo nLocale, LOCALE_SDECIMAL, Value
        End If
    End If
End Property

Option Explicit On 
'Déclaration des fonctions API

Private Declare Function GetLocaleInfo Lib "kernel32" Alias "GetLocaleInfoA" (ByVal Locale As Integer, ByVal LCTYPE As Integer, ByVal lpLCData As String, ByVal cchData As Integer) As Integer

Private Declare Function SetLocaleInfo Lib "kernel32" Alias "SetLocaleInfoA" (ByVal Locale As Integer, ByVal LCTYPE As Integer, ByVal lpLCData As String) As Integer

Private Declare Function GetUserDefaultLCID Lib "kernel32" () As Integer

'Déclaration de la constante séparateur décimal
Private Const LOCALE_SDECIMAL = &HE 
Public Property DecimalSeparator() As String

Get

Dim nLength As Integer

Dim nLocale As Integer

nLocale = GetUserDefaultLCID()

nLength = GetLocaleInfo(nLocale, LOCALE_SDECIMAL, vbNullString, 0) - 1

DecimalSeparator = Space$(nLength)

GetLocaleInfo(nLocale, LOCALE_SDECIMAL, DecimalSeparator, nLength)

End Get
Set(ByVal value As String) 
Dim nLocale As Integer

If value <> DecimalSeparator Then

If value = "." Or value = "," Then

nLocale = GetUserDefaultLCID()

SetLocaleInfo(nLocale, LOCALE_SDECIMAL, value)

End If

End If

End Set

End Property

'Exemple d'utilisation:
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click 
DecimalSeparator = "."

End Sub

' *** A Placer dans un Module
Dim ActualDecimalSeparator As String
Public Sub ChangeDecimalSeparator(ByVal sDecimal As String)
    With Application
        .UseSystemSeparators = False
        ActualDecimalSeparator = .DecimalSeparator
        .DecimalSeparator = sDecimal
    End With
End Sub
Public Sub RestoreDecimalSeparator()
    With Application
        .UseSystemSeparators = True
        .DecimalSeparator = ActualDecimalSeparator
    End With
End Sub
_______________________________________________________
*** EXEMPLE UTILISATION :
Private Sub Workbook_Open()
    Call ChangeDecimalSeparator(".")
End Sub
Private Sub Workbook_BeforeClose(Cancel As Boolean)
    Call RestoreDecimalSeparator
End Sub

' *** A Placer dans un Module
' je pars juste du principe que l'appli est déclaré comme  :    xlApp
Dim ActualDecimalSeparator As String
Public Sub ChangeDecimalSeparator(ByVal sDecimal As String)
    With xlApp
        .UseSystemSeparators = False
        ActualDecimalSeparator = .DecimalSeparator
        .DecimalSeparator = sDecimal
    End With
End Sub
Public Sub RestoreDecimalSeparator()
    With xlApp
        .UseSystemSeparators = True
        .DecimalSeparator = ActualDecimalSeparator
    End With
End Sub
_______________________________________________________
*** EXEMPLE UTILISATION :
Private Sub MybookOpen()
    '  vos déclarations..
    ' après le xlBook.Open(..),  mettre :
    Call ChangeDecimalSeparator(".")
    ' suite de votre traitement
End Sub
Private Sub MybookClose()
    Call RestoreDecimalSeparator
    'à  partir de là, on peut fermer le classeur et quitter l'appli  excel
End Sub

DecimalSeparator := '.';