Translation of AddIn using resource files

Hello,
I have created an addin for which some texts need to be translated. For this translation I have created a resource file (MyTranslations.resx) which contains the English translations. The access of the resource file is internal.
I made a copy of this resource file for the Dutch translations, named MyTranslations.nl.resx
However these Dutch translations are not used, even though the culture of the thread is nl-BE.
Even if I set the culture myself to nl-BE (Thread.CurrentThread.CurrentUICulture = CultureInfo.GetCultureInfo("nl-BE");) the English translations are used.
I tested the same principle in a C# form application and this works fine.

Anyone has an idea? I think the class needs to be set to localizable, but I can't find how.
FYI: the version of NAV is 2009R2, the addin is created in Visual C# 2010 express.

Thanks,
Andy