public class Handle500Erros : HttpRequestProcessor
{
public override void Process(HttpRequestArgs args)
{

    if (args?.Context?.Response.StatusCode == (int)HttpStatusCode.InternalServerError)
    {
        var errorPageItem =
        Sitecore.Context.Database.GetItem(string.Concat(Sitecore.Context.Site.StartPath, Settings.ErrorPage));
        if (errorPageItem!= null && Sitecore.Context.Item.ID!= errorPageItem.ID)
        {
            Sitecore.Context.Item = errorPageItem;
            //var htmlForErrorPageForSpecificlanguage = GetHtmlforErrorItem
            // set response html to htmlForErrorPageForSpecificlanguage
        }
    }
}
}

<customErrors mode="RemoteOnly" xdt:Transform="Replace">
   <error redirect="/error.html" statusCode="500" />
</customErrors>

<rule name="Server Error Rule" stopProcessing="true" enabled="true">
      <match url=".*" />
      <conditions>
        <add input="{URL}" pattern="^/error.html$" />
        <add input="{HTTP_HOST}" pattern="^(admin|www).(mysite1|mysite2|mysite3)?.*.(com|co.uk)" />
      </conditions>
      <action type="Rewrite" url="/errors/{C:2}.html" />
    </rule>