How to make Find 13.x play nice with Find on-premise

If you are using Find (Search & Navigation) 13.x or later with an index located on a Find on-premise instance you need to do the following to achieve full compatibility. 
This is not applicable to any solution where you're using an index hosted by EPiServer.

1. If you are on 13.0.5 or later you need to make sure you upgrade to at least 13.2.4 and add useLegacySorting=true to your episerver.find config element to be
    able to use the Elastic Search sorting functionality to it's full extent.

<episerver.find serviceurl="..." useLegacySorting="true" ... >

2. With Find 13 language routing was introduced. This is not available on Find on-premise therefor you should disable it.
    Disable Language Routing support via LanguageRoutingFactory. 

   [InitializableModule]
   [ModuleDependency(typeof(IndexingModule))]
   public class MyFindInitializationModule : IConfigurableModule
   {
       public void Initialize(InitializationEngine context)
       {
       }
       public void Uninitialize(InitializationEngine context)
       {
       }
       public void ConfigureContainer(ServiceConfigurationContext context)
       {
           context.Services.AddSingleton<LanguageRoutingFactory, MyLanguageRoutingFactory>();
       }
       }

   public class MyLanguageRoutingFactory : LanguageRoutingFactory
   {
       public override LanguageRouting CreateLanguageRouting(ILocale locale)
       {
           return null;
       }
   }
   }