If you don't mind my English, after all, it is an English language site, you could try this.
2 And 3 columns to the right of the last used column need to be free to be used for the result.
Sub Pour_Le_Francais()
Dim lr As Long, lc As Long, j As Long, i As Long
lr = Cells(Rows.Count, 1).End(xlUp).Row
lc = Cells(1, Columns.Count).End(xlToLeft).Column
For j = 2 To lr
For i = 2 To lc
With Cells(Cells(Rows.Count, lc + 2).End(xlUp).Row, lc + 2).Offset(1)
.Value = Cells(j, 1) & "/" & Cells(1, i)
.Offset(, 1).Value = Cells(j, i)
End With
Next i
Next j
End Sub