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We haven't thought for such scenario, so we are not sure if that will work properly.
Once you have set the autoSetClientLanguage="false" you could try to implement an HttpModule, which matches an url with /en/default.aspx and performs redirect to /default.aspx. This may lead to a circle reference though - we haven't tried it here.
Please register the module in the end of the httpModule collection in the web.config file, so the redirect happens after all the other things the CMS is doing.
the Telerik team