public class TranslateAttribute: ActionFilterAttribute, IActionFilter
{
    void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)
    {
        ViewDataDictionary viewData = filterContext.Controller.ViewData;
        if (viewData is CustomViewData) {
            (viewData as CustomViewData).Language = new Language(filterContext.ActionParameters["siteLanguage"] as string));
        }
        base.OnActionExecuting(filterContext);
    }

    void IActionFilter.OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);
    }
}

public class TranslateAttribute: ActionFilterAttribute, IActionFilter
{
    void IActionFilter.OnActionExecuting(ActionExecutingContext filterContext)
    {
        ViewDataDictionary viewData = filterContext.Controller.ViewData;
        if (viewData is CustomViewData) {
            (viewData as CustomViewData).Language = new Language(filterContext.ActionParameters["siteLanguage"] as string));
        }
        base.OnActionExecuting(filterContext);
    }

    void IActionFilter.OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);
    }
}

var customData = new CustomViewData( filterContext.Controller.ViewData);
customData.Language = new Language(filterContext.ActionParameters["siteLanguage"] as string));
filterContext.Controller.ViewData = customData;
base.OnActionExecuting( filterContext );