J'espère que tu as un peu compris comment j'ai procédé : je fais par étapes, exemple, d'abord les billiards, les milliards, les millions... et à chaque fois la fonction getUnite me permet de savoir combien (ex: 15 millions).

Je propose une solution en BASIC (VBA, écrit dans un classeur Excel).
Placez le code ci-dessous dans un module de classeur Excel :

'DEBUT DU CODE

Option Explicit
Option Base 0

Rem  *****  BASIC  *****

Function numToAlpha(x As String, Optional u As Boolean = True, Optional v As String = "") As String
 ' _________________________________ Mémo _________________________________________________________________________________
 ' Écriture en lettres d'un nombre donné en chiffres
 ' Fonction créée le 16/07/2003 par ROGER
 ' Modifiée le 25/07/2008 par ROGER
 ' x est le nombre donné, de 0 à 999 999 999 999 999 999 (on peut essayer plus grand...)
 ' u (VRAI par défaut) contrôle l'écriture de 1000 ("mille" si u=1 ou VRAI ou est omis, "mil" si u=0 ou FAUX)
 ' v ("" par défaut, "d'" ou "de" typiquement) ajoute v à la fin si le nombre se termine par
 ' "milliard(s)" ou "million(s)".
 ' =numToAlpha(1948)        renvoie mille neuf cent quarante-huit
 ' =numToAlpha(1948,9)      renvoie mille neuf cent quarante-huit (la partie décimale est ignorée)
 ' =numToAlpha(1948;1)      renvoie mille neuf cent quarante-huit
 ' =numToAlpha(1948;0)      renvoie mil neuf cent quarante-huit
 ' =numToAlpha("1 000 000";;"de francs")
 ' renvoie  un million de francs
 ' =numToAlpha("876 543 200 980 654 321";1)
 ' renvoie huit cent soixante-seize millions cinq cent quarante-trois mille deux cents milliards
 '      neuf cent quatre-vingts millions six cent cinquante-quatre mille trois cent vingt-et-un
 ' ________________________________________________________________________________________________________________________
    If IsMissing(v) Then v = "" Else v = " " & v
    numToAlpha = NZ312UX(x, u, v)
End Function

Function NZ312UX(x As String, u As Boolean, v As String) As String
 ' ATTENTION : Cette fonction est récursive.
    Dim it As String, y As String
    x = clean1(x)
    On Error GoTo annule
    If Len(x) <> 0 Then
        If x <> "0" Then
            If Len(x) > 9 Then y = Left$(x, Len(x) - 9) Else y = "0"
            If y <> "0" Then
                it = it & NZ312UX(y, u, "") '*** *** *** APPEL RECURSIF DE NZ312UX *** *** ***
                If CDec(Right$(y, 6)) = 0 Then it = it & " de"
                it = it & " milliard": If y <> "1" Then it = it & "s"
            End If
            y = CStr(CDec(Right$(x, 9)))
            If y <> "0" Then
                If it <> "" Then it = it & " "
                it = it & t9(y, u): If CDec(Right$(y, 6)) = 0 Then it = it & v
            Else: it = it & v
            End If
        Else
            it = NZ312UY(0)
        End If
    End If
annule:
    NZ312UX = it
End Function

Function t9(x As String, Optional u As Boolean = True) As String
    Dim it As String, y As String
    y = CStr(CDec("0" & Right$(Left$(x, max(0, Len(x) - 6)), 3)))
    If y <> "0" Then
        it = it & t3(y) & " million": If y <> "1" Then it = it & "s"
    End If
    y = CStr(CDec("0" & Right$(Left$(x, max(0, Len(x) - 3)), 3)))
    If y <> "0" Then
        If it <> "" Then it = it & " "
        If y <> "1" Then it = it & t3(y, 0) & " "
        it = it & "mil": If u Then it = it & "le"
    End If
    y = CStr(CDec(Right$(x, 3)))
    If y <> "0" Then
        If it <> "" Then it = it & " "
        it = it & t3(y)
    End If
    t9 = it
End Function

Function t3(x As String, Optional u As Boolean = True) As String
    Dim it As String, y As String
    If x <> "0" Then
        y = CStr(CDec("0" & Left$(x, max(0, Len(x) - 2)))): x = CDec(Right$(x, 2))
        If y <> "0" Then
            If y <> "1" Then it = NZ312UY(CInt(y)) & " "
            it = it & "cent": If x = "0" And y <> "1" And u Then it = it & "s"
        End If
        If x <> "0" Then
            If it <> "" Then it = it & " "
            it = it & NZ312UY(CInt(x)): If x = "80" And u Then it = it & "s"
        End If
    End If
    t3 = it
End Function

Function clean1(x As String) As String
    Dim it As String, compte As Integer
    For compte = 1 To Len(x)
        If Mid(x, compte, 1) = "," Then Exit For
        If IsNumeric(Mid(x, compte, 1)) And Mid(x, compte, 1) <> " " And Mid(x, compte, 1) <> Chr(160) Then it = it + Mid(x, compte, 1)
    Next compte
    clean1 = it
End Function

Function clean2(x As String) As String
    Dim it As String, compte As Integer
    For compte = 1 To Len(x)
        If (Mid(x, compte, 1) = "," Or IsNumeric(Mid(x, compte, 1))) And Mid(x, compte, 1) <> " " And Mid(x, compte, 1) <> Chr(160) Then it = it + Mid(x, compte, 1)
    Next compte
    clean2 = it
End Function

Function min(a As Long, b As Long) As Long
    min = (a + b - Abs(a - b)) / 2
End Function

Function max(a As Long, b As Long) As Long
    max = (a + b + Abs(a - b)) / 2
End Function

Function NZ312UY(x As Integer) As String
 ' ATTENTION : Cette fonction est récursive.
    Dim a As Variant, s As String
    a = Array("zéro", "un", "deux", "trois", "quatre", "cinq", "six", "sept", "huit", "neuf", "dix", "onze", _
    "douze", "treize", "quatorze", "quinze", "seize", "vingt", "trente", "quarante", "cinquante", _
    "soixante", "quatre-vingt")
    Select Case x
        Case 0 To 16: s = a(x)
        Case 17 To 19: s = a(10 * Int(x / 10)) & "-" & a(x Mod 10)
        Case 20 To 69: s = a(15 + Int(x / 10))
        If x Mod 10 = 1 Then s = s & " et " & a(x Mod 10) Else If x Mod 10 > 1 Then s = s & "-" & a(x Mod 10)
        Case 70 To 99: s = a(18 + Int(x / 20))
        If x = 71 Then s = s & " et " & a(x Mod 20) Else If x Mod 20 > 0 Then s = s & "-" & NZ312UY(x Mod 20)
        Case Else: s = "?"
    End Select
    NZ312UY = s
End Function
 ' ________________________________________________________________________________________________________________________

Function numDecToAlpha(x As String, Optional u As Boolean = True) As String
    Dim c As Integer
    x = clean2(x)
    If Len(x) <> 0 Then
        c = InStr(x, ",")
        If c <> 0 Then
            numDecToAlpha = numToAlpha(Left$(x, c - 1), u) & " virgule "
            Do While InStr(Right$(x, Len(x) - c), "0") = 1
                numDecToAlpha = numDecToAlpha & "zéro ": c = c + 1
            Loop
            numDecToAlpha = numDecToAlpha & numToAlpha(Right$(x, Len(x) - c), u)
        Else
            numDecToAlpha = numToAlpha(x, u)
        End If
    Else
        numDecToAlpha = ""
    End If
End Function

Function FRFA(x As String, Optional u As Boolean = True) As String
    Dim c As Integer
    x = clean2(x)
    If Len(x) <> 0 Then
        c = InStr(x, ",")
        If c <> 0 Then
            FRFA = numToAlpha(Left$(x, c - 1), u, "de")
            FRFA = FRFA & " franc"
            If CDec(x) >= 2 Then FRFA = FRFA & "s"
            FRFA = FRFA & " et " & numToAlpha(Mid$(x & "0", c + 1, 2), u) & " centime"
            If CDec(Mid$(x, c + 1, 2)) >= 2 Then FRFA = FRFA & "s"
        Else
            FRFA = numToAlpha(x, u, "de")
            FRFA = FRFA & " franc"
            If CDec(x) >= 2 Then FRFA = FRFA & "s"
        End If
    Else
        FRFA = ""
    End If
End Function

Function EURA(x As String, Optional u As Boolean = True) As String
    Dim c As Integer
    x = clean2(x)
    If Len(x) <> 0 Then
        c = InStr(x, ",")
        If c <> 0 Then
            EURA = numToAlpha(Left$(x, c - 1), u, "d'")
            If Right$(EURA, 1) = "'" Then EURA = EURA & "euro" Else EURA = EURA & " euro"
            If CDec(x) >= 2 Then EURA = EURA & "s"
            EURA = EURA & " et " & numToAlpha(Mid$(x & "0", c + 1, 2), u) & " centime"
            If CDec(Mid$(x, c + 1, 2)) >= 2 Then EURA = EURA & "s"
        Else
            EURA = numToAlpha(x, u, "d'")
            If Right$(EURA, 1) = "'" Then EURA = EURA & "euro" Else EURA = EURA & " euro"
            If CDec(x) >= 2 Then EURA = EURA & "s"
        End If
    Else
        EURA = ""
    End If
End Function
 ' 

TEST LA CATCHEUSE