Hi Tarek,
Sounds like it should be simple, but hasn't proved to be. The closest I've come is to edit the XML to look like;
<calculate override="error">
<message>
<text name="override">Some french transation</text>
</message>
<script contentType="application/x-javascript">"value"</script>
</calculate>
This doesn't remove the English error but does let you add a French one above it.
Regards
Bruce