Dear jonathanexact,
You can set the CustomCulture to "nl-NL"
<CE:Editor id="Editor1" CustomCulture="nl-NL" ThemeType="Office2007" runat="server" AutoConfigure="Simple" ></CE:Editor><br />
If you don't want to change the CustomCulture in CuteEditor and only want to modify the spell check language, you can modify file "\CuteSoft_Client\CuteEditor\SpellCheck.aspx" directly. Change the following line:
culture = Request.Params["Culture"];
to
culture = "nl-NL";
HTML & scripts are checked as well. This issue, we will provide solution in future version.
Thank you for asking