Excel Add-In not working in French Excel

I have developed an Add-in for excel using VB .NET and ExcelDNA. The Add-In works on my computer and all computers which has an english version of excel. However, it does not work on my clients computer(is in English) which has a French version of Excel.

I get the following error:

Old Format or invalid type library HResult 0x80028018

I have spent hours on the internet and the solution I found was that you need to change the Culture Info. I added this to my code does not seem to work,

        Dim oldCI As System.Globalization.CultureInfo = _
        System.Threading.Thread.CurrentThread.CurrentCulture
        System.Threading.Thread.CurrentThread.CurrentCulture = _
         New System.Globalization.CultureInfo("fr-FR")
        System.Threading.Thread.CurrentThread.CurrentCulture = oldCI

Well, the only solution I found was to convert my whole project to .NET 4.0, and now it works fine.