Roy,
Lets be clear about what you are trying to do. You want to change your test machine's local and have TestComplete and the webapplication act accordingly ?
I am not sure that it is the right approach. As you are testing a web application, you might want to check into the browser's language settings (attached screenshot). It would be much easier to simulate locals that way.
In any case, that does not change localization is handled by TC. If you use NameMapping, you might want to look into Name Mapping configuration (
here).
Sincerely
