In your french workbook perhaps:
Code:
=IFERROR(VALUE(B3);VALUE(SUBSTITUTE(B3;".";",")))
I
think the formula semicolons automatically get changed to commas if opening it in an English version of Excel.
edit:
Or perhaps:
Code:
=SIERREUR(CNUM(B3);CNUM(SUBSTITUE(B3;".";",")))